| Message ID | 20200130225818.193825-1-masayasuzuki@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | doc: describe Git bundle format | expand |
Hi, On Thu, 30 Jan 2020, Masaya Suzuki wrote: > The bundle format was not documented. Describe the format with ABNF and > explain the meaning of each part. LGTM, Dscho > > Signed-off-by: Masaya Suzuki <masayasuzuki@google.com> > --- > Documentation/technical/bundle-format.txt | 40 +++++++++++++++++++++++ > 1 file changed, 40 insertions(+) > create mode 100644 Documentation/technical/bundle-format.txt > > diff --git a/Documentation/technical/bundle-format.txt b/Documentation/technical/bundle-format.txt > new file mode 100644 > index 0000000000..dbb80225b5 > --- /dev/null > +++ b/Documentation/technical/bundle-format.txt > @@ -0,0 +1,40 @@ > += Git bundle v2 format > + > +The Git bundle format is a format that represents both refs and Git objects. > + > +== Format > + > +We will use ABNF notation to define the Git bundle format. See > +protocol-common.txt for the details. > + > +---- > +bundle = signature references pack > +signature = "# v2 git bundle" LF > + > +references = *(prerequisite / ref) LF > +prerequisite = "-" obj-id SP comment LF > +comment = *CHAR > +ref = obj-id SP refname LF > + > +pack = ... ; packfile > +---- > + > +== Semantics > + > +A Git bundle consists of three parts. > + > +* Prerequisites: Optional list of objects that are not included in the bundle > + file. A bundle can reference these prerequisite objects (or it can reference > + the objects reachable from the prerequisite objects). The bundle itself > + might not contain those objects. > +* References: Mapping of ref names to objects. > +* Git objects: Commit, tree, blob, and tags. These are included in the pack > + format. > + > +If a bundle contains prerequisites, it means the bundle has a thin pack and the > +bundle alone is not enough for resolving all objects. When you read such > +bundles, you should have those missing objects beforehand. > + > +In the bundle format, there can be a comment following a prerequisite obj-id. > +This is a comment and it has no specific meaning. When you write a bundle, you > +can put any string here. When you read a bundle, you can ignore this part. > -- > 2.25.0.341.g760bfbb309-goog > >
Masaya Suzuki <masayasuzuki@google.com> writes: > The bundle format was not documented. Describe the format with ABNF and > explain the meaning of each part. Thanks. > > Signed-off-by: Masaya Suzuki <masayasuzuki@google.com> > --- > Documentation/technical/bundle-format.txt | 40 +++++++++++++++++++++++ > 1 file changed, 40 insertions(+) > create mode 100644 Documentation/technical/bundle-format.txt > > diff --git a/Documentation/technical/bundle-format.txt b/Documentation/technical/bundle-format.txt > new file mode 100644 > index 0000000000..dbb80225b5 > --- /dev/null > +++ b/Documentation/technical/bundle-format.txt > @@ -0,0 +1,40 @@ > += Git bundle v2 format > + > +The Git bundle format is a format that represents both refs and Git objects. > + > +== Format > + > +We will use ABNF notation to define the Git bundle format. See > +protocol-common.txt for the details. > + > +---- > +bundle = signature references pack > +signature = "# v2 git bundle" LF Good. "signature" is the name used by bundle.c::create_bundle() to call this part. > +references = *(prerequisite / ref) LF This allows prereq and ref can come inter-mixed, but I think we show all prerequisites first before refs. > +prerequisite = "-" obj-id SP comment LF > +comment = *CHAR Do readers know what CHAR consists of? Anything other than NUL and LF? > +ref = obj-id SP refname LF OK. "prerequisite" and "ref" are both used in bundle.c::create_bundle(), so calling these parts with these names is consistent with the code. "head" is also a good name for the latter as "git bundle list-heads" is the way the end-users access them from outside. > + > +pack = ... ; packfile > +---- > + > +== Semantics > + > +A Git bundle consists of three parts. > + > +* Prerequisites: Optional list of objects that are not included in the bundle > + file. A bundle can reference these prerequisite objects (or it can reference > + the objects reachable from the prerequisite objects). The bundle itself > + might not contain those objects. While not incorrect per-se, the above misses the more important points (and defers the description to a later paragraph). It is better to describe what it means to have prereqs upfront. > +* References: Mapping of ref names to objects. > +* Git objects: Commit, tree, blob, and tags. These are included in the pack > + format. > + Match the name you used to descibe the parts in the earlier ABNF description, so that the correspondence is clear to the readers. You somehow used "references" to mean both prereqs and heads, but in the above you are describing only "heads" under the label of "references". Perhaps something like this? * "Prerequisites" lists the objects that are NOT included in the bundle and the receiver of the bundle MUST already have, in order to use the data in the bundle. The objects stored in the bundle may refer to prerequiste objects and anything reachable from them and/or expressed as a delta against prerequisite objects. * "Heads" record the tips of the history graph, iow, what the receiver of the bundle CAN "git fetch" from it. * "Pack" is the pack data stream "git fetch" would send, if you fetch from a repository that has the references recorded in the "Heads" above into a repository that has references pointing at the objects listed in "Prerequisites" above. > +If a bundle contains prerequisites, it means the bundle has a thin pack and the > +bundle alone is not enough for resolving all objects. When you read such > +bundles, you should have those missing objects beforehand. With the above rewrite, this paragraph is unneeded. > +In the bundle format, there can be a comment following a prerequisite obj-id. > +This is a comment and it has no specific meaning. When you write a bundle, you > +can put any string here. When you read a bundle, you can ignore this part. Is it "you can"? At least the last one should be "readers of a bundle MUST ignore the comment", I think.
On Fri, Jan 31, 2020 at 12:38 PM Junio C Hamano <gitster@pobox.com> wrote: > > Masaya Suzuki <masayasuzuki@google.com> writes: > > > The bundle format was not documented. Describe the format with ABNF and > > explain the meaning of each part. > > Thanks. > > > > > Signed-off-by: Masaya Suzuki <masayasuzuki@google.com> > > --- > > Documentation/technical/bundle-format.txt | 40 +++++++++++++++++++++++ > > 1 file changed, 40 insertions(+) > > create mode 100644 Documentation/technical/bundle-format.txt > > > > diff --git a/Documentation/technical/bundle-format.txt b/Documentation/technical/bundle-format.txt > > new file mode 100644 > > index 0000000000..dbb80225b5 > > --- /dev/null > > +++ b/Documentation/technical/bundle-format.txt > > @@ -0,0 +1,40 @@ > > += Git bundle v2 format > > + > > +The Git bundle format is a format that represents both refs and Git objects. > > + > > +== Format > > + > > +We will use ABNF notation to define the Git bundle format. See > > +protocol-common.txt for the details. > > + > > +---- > > +bundle = signature references pack > > +signature = "# v2 git bundle" LF > > Good. "signature" is the name used by bundle.c::create_bundle() to > call this part. > > > +references = *(prerequisite / ref) LF > > This allows prereq and ref can come inter-mixed, but I think we show > all prerequisites first before refs. Based on bundle.c::parse_bundle_header(), I infer that this can be mixed. If that's not intended, this can be changed to have prerequisites first. > > > +prerequisite = "-" obj-id SP comment LF > > +comment = *CHAR > > Do readers know what CHAR consists of? Anything other than NUL and > LF? RFC 5234 defines core rules (https://tools.ietf.org/html/rfc5234#appendix-B.1), and these CHAR etc are defined there. It should be OK to use these rules. > > > +ref = obj-id SP refname LF > > OK. > > "prerequisite" and "ref" are both used in bundle.c::create_bundle(), > so calling these parts with these names is consistent with the code. > "head" is also a good name for the latter as "git bundle list-heads" > is the way the end-users access them from outside. > > > + > > +pack = ... ; packfile > > +---- > > + > > +== Semantics > > + > > +A Git bundle consists of three parts. > > + > > +* Prerequisites: Optional list of objects that are not included in the bundle > > + file. A bundle can reference these prerequisite objects (or it can reference > > + the objects reachable from the prerequisite objects). The bundle itself > > + might not contain those objects. > > While not incorrect per-se, the above misses the more important > points (and defers the description to a later paragraph). It is > better to describe what it means to have prereqs upfront. > > > +* References: Mapping of ref names to objects. > > +* Git objects: Commit, tree, blob, and tags. These are included in the pack > > + format. > > + > > Match the name you used to descibe the parts in the earlier ABNF > description, so that the correspondence is clear to the readers. > You somehow used "references" to mean both prereqs and heads, but in > the above you are describing only "heads" under the label of > "references". Yes. It should match with the ABNF definition above. I usually use "heads" to mean "references under refs/heads/*" (not sure if this is true for other people). Since a bundle can contain tags etc., using "heads" here seems confusing. With prerequisites and references split you mentioned above, I think I can make ABNF and this semantics section consistent in terms of wording. bundle = signature *prerequisite *ref LF pack prerequisite = "-" obj-id SP comment LF comment = *CHAR reference = obj-id SP refname LF pack = ... ; packfile The terms ("prerequisite" and "reference") are consistent with bundle.h::ref_list. > > Perhaps something like this? > > * "Prerequisites" lists the objects that are NOT included in the > bundle and the receiver of the bundle MUST already have, in > order to use the data in the bundle. The objects stored in > the bundle may refer to prerequiste objects and anything > reachable from them and/or expressed as a delta against > prerequisite objects. I want to make sure the meaning of prerequisites. 1. Are they meant for a delta base? Or are they meant to represent a partial/shallow state? If these prerequisites are used as a delta base, the receiver of the bundle MUST have them. If these prerequisites are the indicators of the shallowness or the partialness of the repository, the pack data would have complete data in terms of deltification (e.g. all objects in the pack file can be undeltified with just the pack file), and the bundle can be treated as a shallow-cloned/partially-cloned repository snapshot. From what I can see from bundle.c, I think it's an indicator of a delta base, not an indicator of a shallow/partial state, but I want to make sure. 2. Do they need to be commits? Or can they be any object type? From what I can see, it seems that they should always be commits. 3. Does the receiver have to have all reachable objects from prerequisites? My understanding is "Yes, the receiver must have all reachable objects from prerequisites." This means that if a receiver has a shallow-cloned repository, they might not be able to proceess a bundle with prerequisites. The bundle's pack part can deltify against the objects that exist beyond the shallow depth. > > * "Heads" record the tips of the history graph, iow, what the > receiver of the bundle CAN "git fetch" from it. > > * "Pack" is the pack data stream "git fetch" would send, if you > fetch from a repository that has the references recorded in > the "Heads" above into a repository that has references > pointing at the objects listed in "Prerequisites" above. I'll adopt this in the next patch. > > > +If a bundle contains prerequisites, it means the bundle has a thin pack and the > > +bundle alone is not enough for resolving all objects. When you read such > > +bundles, you should have those missing objects beforehand. > > With the above rewrite, this paragraph is unneeded. > > > +In the bundle format, there can be a comment following a prerequisite obj-id. > > +This is a comment and it has no specific meaning. When you write a bundle, you > > +can put any string here. When you read a bundle, you can ignore this part. > > Is it "you can"? At least the last one should be "readers of a > bundle MUST ignore the comment", I think. I'll change this to MUST.
Masaya Suzuki <masayasuzuki@google.com> writes: >> > +prerequisite = "-" obj-id SP comment LF >> > +comment = *CHAR >> >> Do readers know what CHAR consists of? Anything other than NUL and >> LF? > > RFC 5234 defines core rules > (https://tools.ietf.org/html/rfc5234#appendix-B.1), and these CHAR etc > are defined there. It should be OK to use these rules. That's not what I asked. Do readers know that? Did you tell them that we expect they are familiar with the RFC convention? It might be easier to make the above simple ABNF understandable to those without knowledge of RFC 5234 by spelling out what CHAR in the context of the above description means. Or to tell them "go over there and learn CHAR then come back". We need to do one of them. > I want to make sure the meaning of prerequisites. > > 1. Are they meant for a delta base? Or are they meant to represent a > partial/shallow state? They are meant as the "bottom boundary" of the range of the pack data stored in the bundle. Think of "git rev-list --objects $heads --not $prerequisites". If we limit ourselves to commits, in the simplest case, "git log maint..master". Imagine your repository has everything up to 'maint' (and nothing else) and then you are "git fetch"-ing from another repository that advanced the tip that now points at 'master'. Imagine the data transferred over the network. Imagine that data is frozen on disk somehow. That is what a bundle is. So, 'maint' is the prerequisite---for the person who builds the bundle, it can safely be assumed that the bundle will be used only by those who already has 'maint'. There is nothing about 'partial' or 'shallow'. And even though a bundle typically has deltified objects in the packfile, it does not have to. Some objects are delitifed against prerequisite, and the logic to generate thin packs may even prefer to use the prerequisites as the delta base, but it is merely a side effect that the prerequisites are at the "bottom boundary" of the range. > 2. Do they need to be commits? Or can they be any object type? > > From what I can see, it seems that they should always be commits. > > 3. Does the receiver have to have all reachable objects from prerequisites? I would say that the receiver needs to have everything that is needed to "complete" prereqs. Bundle transfer predates shallow or incomplete repositories, but I think that we can (and we should if needed) update it to adjust to these situations by using the appropriate definition of what it means to "complete". In a lazy clone, it may be sufficient to have promisor remote that has everything reachable from them. In a shallow clone, the repository may have to be deep enough to have them and objects immediately reachable from them (e.g. trees and blobs for a commit at the "bottom boundary"). Thanks.
On Fri, Jan 31, 2020 at 3:01 PM Junio C Hamano <gitster@pobox.com> wrote: > > Masaya Suzuki <masayasuzuki@google.com> writes: > > >> > +prerequisite = "-" obj-id SP comment LF > >> > +comment = *CHAR > >> > >> Do readers know what CHAR consists of? Anything other than NUL and > >> LF? > > > > RFC 5234 defines core rules > > (https://tools.ietf.org/html/rfc5234#appendix-B.1), and these CHAR etc > > are defined there. It should be OK to use these rules. > > That's not what I asked. Do readers know that? Did you tell them > that we expect they are familiar with the RFC convention? The patch says "We will use ABNF notation to define the Git bundle format. See protocol-common.txt for the details.", and protocol-common.txt says "ABNF notation as described by RFC 5234 is used within the protocol documents, except the following replacement core rules are used:". In order to interpret this ABNF definition, it's not enough to read RFC 5234, but the reader has to read protocol-common.txt. Otherwise, they cannot understand what `obj-id` is and what `refname` is. Those are not defined in RFC 5234. They're defined in protocol-common.txt. Based on the fact that (1) this document instructs the reader to see protocol-common.txt in the beginning and (2) protocol-common.txt is needed to interpret this definition and protocol-common.txt says RFC 5234 describes ABNF format, the readers should know ABNF is defined in RFC 5234 and ABNF includes those LF, CHAR, and SP as a part of the definition after reading the first sentence and referenced documents. > > It might be easier to make the above simple ABNF understandable to > those without knowledge of RFC 5234 by spelling out what CHAR in the > context of the above description means. Or to tell them "go over > there and learn CHAR then come back". We need to do one of them. As I said above, the first sentence says "See protocol-common.txt" which includes the reference to the RFC and other non-terminals. Note that, not only CHAR, but obj-id and refname are not defined here as well. The readers need to reference protocol-common.txt to get the definition of them. > > > I want to make sure the meaning of prerequisites. > > > > 1. Are they meant for a delta base? Or are they meant to represent a > > partial/shallow state? > > They are meant as the "bottom boundary" of the range of the pack > data stored in the bundle. > > Think of "git rev-list --objects $heads --not $prerequisites". If > we limit ourselves to commits, in the simplest case, "git log > maint..master". Imagine your repository has everything up to > 'maint' (and nothing else) and then you are "git fetch"-ing from > another repository that advanced the tip that now points at > 'master'. Imagine the data transferred over the network. Imagine > that data is frozen on disk somehow. That is what a bundle is. > > So, 'maint' is the prerequisite---for the person who builds the > bundle, it can safely be assumed that the bundle will be used only > by those who already has 'maint'. > > There is nothing about 'partial' or 'shallow'. And even though a > bundle typically has deltified objects in the packfile, it does not > have to. Some objects are delitifed against prerequisite, and the > logic to generate thin packs may even prefer to use the > prerequisites as the delta base, but it is merely a side effect that > the prerequisites are at the "bottom boundary" of the range. OK. Then, it's better to make this clear. If you follow the analogy of saved git-fetch response, it's possible that these prerequisites are interpreted same as "shallow" lines of the shallow clone response. It's more like "have" lines of git-fetch request. > > 2. Do they need to be commits? Or can they be any object type? > > > > From what I can see, it seems that they should always be commits. > > > > 3. Does the receiver have to have all reachable objects from prerequisites? > > I would say that the receiver needs to have everything that is > needed to "complete" prereqs. > > Bundle transfer predates shallow or incomplete repositories, but I > think that we can (and we should if needed) update it to adjust to > these situations by using the appropriate definition of what it > means to "complete". In a lazy clone, it may be sufficient to have > promisor remote that has everything reachable from them. In a > shallow clone, the repository may have to be deep enough to have > them and objects immediately reachable from them (e.g. trees and > blobs for a commit at the "bottom boundary"). I think there are two completeness of a packfile: * Delta complete: If an object in a packfile is deltified, the delta base exists in the same packfile. * Object complete: If an object in a packfile contains a reference to another object, that object exists in the same packfile. For example, initial shallow clone response should contain a delta-complete object-incomplete packfile. Incremental fetch response and bundles with prereqs would have a delta-incomplete object-incomplete packfile. Creating delta-incomplete object-complete packfile is possible (e.g. create a parallel history with all blobs slightly modified and deltify against the original branch. I can create a packfile with all objects in one history with all objects deltified with the other history), but it's a rare case. The reader of a bundle SHOULD have all objects reachable from prereqs.
Masaya Suzuki <masayasuzuki@google.com> writes: > * Delta complete: If an object in a packfile is deltified, the delta > base exists in the same packfile. Yes, even though "thin" packs delierately violate this to save size, normal packs, and more importantly, on-disk packs, are complete in this sense. > * Object complete: If an object in a packfile contains a reference to > another object, that object exists in the same packfile. A single packfile that would result from a full clone at some time in the project's history would be "complete" in this sense. Such a packfile may contain all objects that are needed to reproduce the history up to v1.0.1, or another larger "object complete" packfile may contain everything needed for the history up to v3.0.0. So as a concept, this can be defined sensibly. In the original packfile design, however, this concept was not useful (iow, there was nowhere that cared if a packfile is "object complete" or not), so I do not think there is no explicit "support" to ensure or validate this trait in the system. Obviously, a bundle that stores object incomplete pack must have been created with the bottom boundary. > The reader of a bundle SHOULD have all objects reachable from prereqs. Perhaps. It _might_ be possible to teach "git clone" to produce a shallow clone whose shallow cut-off points match the prerequisites of the bundle, so it depends on what the reader wants to do with the data, though. Thanks.
diff --git a/Documentation/technical/bundle-format.txt b/Documentation/technical/bundle-format.txt new file mode 100644 index 0000000000..dbb80225b5 --- /dev/null +++ b/Documentation/technical/bundle-format.txt @@ -0,0 +1,40 @@ += Git bundle v2 format + +The Git bundle format is a format that represents both refs and Git objects. + +== Format + +We will use ABNF notation to define the Git bundle format. See +protocol-common.txt for the details. + +---- +bundle = signature references pack +signature = "# v2 git bundle" LF + +references = *(prerequisite / ref) LF +prerequisite = "-" obj-id SP comment LF +comment = *CHAR +ref = obj-id SP refname LF + +pack = ... ; packfile +---- + +== Semantics + +A Git bundle consists of three parts. + +* Prerequisites: Optional list of objects that are not included in the bundle + file. A bundle can reference these prerequisite objects (or it can reference + the objects reachable from the prerequisite objects). The bundle itself + might not contain those objects. +* References: Mapping of ref names to objects. +* Git objects: Commit, tree, blob, and tags. These are included in the pack + format. + +If a bundle contains prerequisites, it means the bundle has a thin pack and the +bundle alone is not enough for resolving all objects. When you read such +bundles, you should have those missing objects beforehand. + +In the bundle format, there can be a comment following a prerequisite obj-id. +This is a comment and it has no specific meaning. When you write a bundle, you +can put any string here. When you read a bundle, you can ignore this part.
The bundle format was not documented. Describe the format with ABNF and explain the meaning of each part. Signed-off-by: Masaya Suzuki <masayasuzuki@google.com> --- Documentation/technical/bundle-format.txt | 40 +++++++++++++++++++++++ 1 file changed, 40 insertions(+) create mode 100644 Documentation/technical/bundle-format.txt