DEBUG: connection: mark the closed FDs with a value that is easier to detect
Setting an FD to -1 when closed isn't the most easily noticeable thing
to do when we're chasing accidental reuse of a stale file descriptor.
Instead set it to that large a negative value that it will overflow the
fdtab and provide an analysable core at the moment the issue happens.
Care was taken to ensure it doesn't overflow nor change sign on 32-bit
machines when multiplied by fdtab, and that it also remains negative for
the various checks that exist. The value equals 0xFDDEADFD which happens
to be easily spotted in a debugger.
diff --git a/include/types/fd.h b/include/types/fd.h
index 8299a02..7f63093 100644
--- a/include/types/fd.h
+++ b/include/types/fd.h
@@ -76,6 +76,19 @@
FD_ST_READY
};
+
+/* This is the value used to mark a file descriptor as dead. This value is
+ * negative, this is important so that tests on fd < 0 properly match. It
+ * also has the nice property of being highly negative but not overflowing
+ * nor changing sign on 32-bit machines when multipled by sizeof(fdtab).
+ * This ensures that any unexpected dereference of such an uninitialized
+ * file descriptor will lead to so large a dereference that it will crash
+ * the process at the exact location of the bug with a clean stack trace
+ * instead of causing silent manipulation of other FDs. And it's readable
+ * when found in a dump.
+ */
+#define DEAD_FD_MAGIC 0xFDDEADFD
+
/* info about one given fd */
struct fdtab {
void (*iocb)(int fd); /* I/O handler */