spawn: Fix error pipe write failure handling

A short history lesson: this code (from exec.c) used to just be

    write(...);

In 649d85b, an empty if block was added to placate _FORTIFY_SOURCE's
warn_unused_result:

    if (write(...) != sizeof(error)) {
            // ...
    }

This is fine since the parent doesn't rely on receiving those writes
(e.g. the child could die before writing), and the small write to a pipe
won't fail anyway.

But when bfs_spawn() was created, the implementation changed to this:

    while (write(...) < sizeof(error));

The while loop was presumably added to mirror musl's posix_spawn()
implementation, which handles EINTR.  But musl's is actually correct:

    while (write(...) < 0);

whereas mine has a subtle bug: since sizeof(error) is unsigned, the bfs
code did an unsigned comparison, meaning -1 from write() would *not*
restart the loop.

Fix it by comparing with 0 like musl, since short writes are impossible
here.  Perhaps it's time for -Wsign-compare, despite the other 18
instances being false positives.

1 files changed, 8 insertions, 1 deletions

diff --git a/spawn.c b/spawn.c
index b356c41..8c360d3 100644
--- a/spawn.c
+++ b/spawn.c
@@ -190,7 +190,14 @@ static void bfs_spawn_exec(const char *exe, const struct bfs_spawn *ctx, char **
 
 fail:
 	error = errno;
-	while (write(pipefd[1], &error, sizeof(error)) < sizeof(error));
+
+	while (write(pipefd[1], &error, sizeof(error)) < 0) {
+		if (errno != EINTR) {
+			// Parent will still see that we exited unsuccessfully, but won't know why
+			break;
+		}
+	}
+
 	close(pipefd[1]);
 	_Exit(127);
 }