[v2,08/11] reftable/stack: fix use of unseeded randomness

Commit Message

Patrick Steinhardt Dec. 8, 2023, 2:53 p.m. UTC

When writing a new reftable stack, Git will first create the stack with
a random suffix so that concurrent updates will not try to write to the
same file. This random suffix is computed via a call to rand(3P). But we
never seed the function via srand(3P), which means that the suffix is in
fact always the same.

Fix this bug by using `git_rand()` instead, which does not need to be
initialized. While this function is likely going to be slower depending
on the platform, this slowness should not matter in practice as we only
use it when writing a new reftable stack.

Signed-off-by: Patrick Steinhardt <[e-mail]>
---
 reftable/readwrite_test.c | 6 +++---
 reftable/stack.c          | 2 +-
 2 files changed, 4 insertions(+), 4 deletions(-)

Comments

Han-Wen Nienhuys Dec. 21, 2023, 10:49 a.m. UTC | #1

On Fri, Dec 8, 2023 at 3:53 PM Patrick Steinhardt <[e-mail]> wrote:
>
> When writing a new reftable stack, Git will first create the stack with
> a random suffix so that concurrent updates will not try to write to the

I had noticed this already. Thanks for fixing!

Patch

diff --git a/reftable/readwrite_test.c b/reftable/readwrite_test.c
index 469ab79a5a..278663f22d 100644
--- a/reftable/readwrite_test.c
+++ b/reftable/readwrite_test.c
@@ -141,8 +141,8 @@  static void test_log_buffer_size(void)
 	*/
 	uint8_t hash1[GIT_SHA1_RAWSZ], hash2[GIT_SHA1_RAWSZ];
 	for (i = 0; i < GIT_SHA1_RAWSZ; i++) {
-		hash1[i] = (uint8_t)(rand() % 256);
-		hash2[i] = (uint8_t)(rand() % 256);
+		hash1[i] = (uint8_t)(git_rand() % 256);
+		hash2[i] = (uint8_t)(git_rand() % 256);
 	}
 	log.value.update.old_hash = hash1;
 	log.value.update.new_hash = hash2;
@@ -320,7 +320,7 @@  static void test_log_zlib_corruption(void)
 	};
 
 	for (i = 0; i < sizeof(message) - 1; i++)
-		message[i] = (uint8_t)(rand() % 64 + ' ');
+		message[i] = (uint8_t)(git_rand() % 64 + ' ');
 
 	reftable_writer_set_limits(w, 1, 1);
 
diff --git a/reftable/stack.c b/reftable/stack.c
index 2f1494aef2..95963f67a2 100644
--- a/reftable/stack.c
+++ b/reftable/stack.c
@@ -434,7 +434,7 @@  int reftable_stack_add(struct reftable_stack *st,
 static void format_name(struct strbuf *dest, uint64_t min, uint64_t max)
 {
 	char buf[100];
-	uint32_t rnd = (uint32_t)rand();
+	uint32_t rnd = (uint32_t)git_rand();
 	snprintf(buf, sizeof(buf), "0x%012" PRIx64 "-0x%012" PRIx64 "-%08x",
 		 min, max, rnd);
 	strbuf_reset(dest);