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[BUG] git_xdiff ignores xdl_diff errors and returns an empty diff #7420
Description
libgit2 commit 07b89d1 src/libgit2/diff_xdiff.c:226.
Bug
git_xdiff discards the return value of xdl_diff and returns only the error recorded by the hunk/line callbacks:
xdl_diff(&info.xd_old_data, &info.xd_new_data,
&xo->params, &xo->config, &xo->callback); /* 226: result ignored */
git_diff_find_context_clear(&findctxt);
return xo->output.error; /* only set by callbacks */When xdl_diff fails (for example out of memory while preparing the diff environment or building the edit script),
it returns -1 before any callback runs.
xo->output.error stays 0, so git_patch_from_buffers and every other text diff consumer (patch, diff, blame, merge) report success with no hunks.
There is no crash.
Reproduce
05.c installs a counting allocator through GIT_OPT_SET_ALLOCATOR that returns NULL for the Nth allocation, which looks exactly like a real OOM to libgit2 (xdl_malloc is git__malloc). It diffs two small buffers with git_patch_from_buffers, once normally and then with the Nth allocation failing, for N = 1..300, and prints every run that returns 0 with a different hunk count.
gcc -g -O0 05.c -I"$LIBGIT2_INCLUDE" "$LIBGIT2_LIBDIR/libgit2.a" -lz -lpthread -ldl -o 05_c
./05_creference: git_patch_from_buffers -> 0, hunks = 1
alloc #4: git_patch_from_buffers -> 0, hunks = 0 (expected 1)
alloc #5: git_patch_from_buffers -> 0, hunks = 0 (expected 1)
...
alloc #26: git_patch_from_buffers -> 0, hunks = 0 (expected 1)
5 clean errors, 23 wrong results reported as success
All 23 wrong results fail inside xdl_diff (xdl_prepare_env, xdl_do_diff, xdl_build_script). The 5 clean errors come from allocations before xdl_diff and from the hunk/line callbacks, whose errors git_xdiff does return.
05.c
#include <git2.h>
#include <git2/sys/alloc.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
static long fail_at, count; /* fail the fail_at-th allocation; 0 = never */
static int should_fail(void) { return fail_at && ++count == fail_at; }
static void *x_malloc(size_t n, const char *file, int line)
{
(void)file; (void)line;
return should_fail() ? NULL : malloc(n ? n : 1);
}
static void *x_realloc(void *p, size_t n, const char *file, int line)
{
(void)file; (void)line;
return should_fail() ? NULL : realloc(p, n ? n : 1);
}
/* Diff OLD against NEW with the nth allocation failing; returns rc, sets *hunks. */
static int diff(long n, size_t *hunks)
{
static const char OLD[] = "alpha\nbeta\ngamma\ndelta\n";
static const char NEW[] = "alpha\nBETA\ngamma\nDELTA\nepsilon\n";
git_patch *patch;
int rc;
fail_at = n, count = 0;
rc = git_patch_from_buffers(&patch, OLD, strlen(OLD), NULL, NEW, strlen(NEW), NULL, NULL);
fail_at = 0;
if (rc == 0) {
*hunks = git_patch_num_hunks(patch);
git_patch_free(patch);
}
return rc;
}
int main(void)
{
git_allocator alloc = { x_malloc, x_realloc, free };
size_t good, hunks;
int wrong = 0, clean = 0;
long n;
setvbuf(stdout, NULL, _IONBF, 0);
if (git_libgit2_init() < 0 || git_libgit2_opts(GIT_OPT_SET_ALLOCATOR, &alloc) ||
diff(0, &good) || good == 0)
return fprintf(stderr, "05_c: setup failed\n"), 1;
printf("reference: git_patch_from_buffers -> 0, hunks = %zu\n", good);
for (n = 1; n <= 300; n++) {
if (diff(n, &hunks))
clean++;
else if (hunks != good)
wrong++, printf("alloc #%ld: git_patch_from_buffers -> 0, hunks = %zu (expected %zu)\n",
n, hunks, good);
}
printf("%d clean errors, %d wrong results reported as success\n", clean, wrong);
return wrong ? 0 : 1;
}
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