[PATCH v2 2/2] Silence buildbot warnings about unused results

[Austin Clements <amdragon@MIT.EDU>]

This ignores the results of the two writes in sigint handlers even
harder than before.

While my libc lacks the declarations that trigger these warnings, this
can be tested by adding the following to notmuch.h:

__attribute__((warn_unused_result))
ssize_t write(int fd, const void *buf, size_t count);
---
 compat/compat.h |    8 ++++++++
 notmuch-new.c   |    6 +++++-
 notmuch-tag.c   |    5 ++++-
 3 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/compat/compat.h b/compat/compat.h
index 7767fe8..b2e2736 100644
--- a/compat/compat.h
+++ b/compat/compat.h
@@ -46,6 +46,14 @@ getdelim (char **lineptr, size_t *n, int delimiter, FILE *fp);
 char* strcasestr(const char *haystack, const char *needle);
 #endif /* !HAVE_STRCASESTR */
 
+/* Silence gcc warnings about unused results.  These warnings exist
+ * for a reason; any use of this needs to be justified. */
+#ifdef __GNUC__
+#define IGNORE_RESULT(x) ({ __typeof__(x) __z = (x); (void)(__z = __z); })
+#else /* !__GNUC__ */
+#define IGNORE_RESULT(x) x
+#endif /* __GNUC__ */
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/notmuch-new.c b/notmuch-new.c
index 3512de7..a569a54 100644
--- a/notmuch-new.c
+++ b/notmuch-new.c
@@ -67,7 +67,11 @@ handle_sigint (unused (int sig))
 {
     static char msg[] = "Stopping...         \n";
 
-    (void) write(2, msg, sizeof(msg)-1);
+    /* This write is "opportunistic", so it's okay to ignore the
+     * result.  It is not required for correctness, and if it does
+     * fail or produce a short write, we want to get out of the signal
+     * handler as quickly as possible, not retry it. */
+    IGNORE_RESULT (write (2, msg, sizeof(msg)-1));
     interrupted = 1;
 }
 
diff --git a/notmuch-tag.c b/notmuch-tag.c
index 292c5da..1564182 100644
--- a/notmuch-tag.c
+++ b/notmuch-tag.c
@@ -26,7 +26,10 @@ static void
 handle_sigint (unused (int sig))
 {
     static char msg[] = "Stopping...         \n";
-    (void) write(2, msg, sizeof(msg)-1);
+
+    /* See handle_sigint in notmuch-new.c for the justification for
+     * ignoring write's result. */
+    IGNORE_RESULT (write (2, msg, sizeof(msg)-1));
     interrupted = 1;
 }
 
-- 
1.7.7.3