lib: Fix potential crashes when writing to log fails with EAGAIN

The ioloop may nowadays call ioloop context switch callbacks. Since log
writing can happen just about anywhere, the callbacks may be confused
and cause crashes or other weird behavior.

Even if the callbacks aren't called, all the extra code in ioloop can cause
potential problems. Especially any error logging in it wouldn't work properly
since it would just recurse back. So replace the ioloop code with just setting
the log fd to be blocking until the write succeeds.

This commit also removes comments about writes to a blocking terminal fd
causing EAGAINs. This seems unlikely. Probably I was just somehow
confused when originally seeing it and writing the code. If it actually
does happen now, it's still not breaking anything, but it could get into
a busy-loop of write()s constantly returning EAGAIN until they succeed.

src/lib/failures.c

@@ -105,20 +105,17 @@ static void log_prefix_add(const struct failure_context *ctx, string_t *str)
 		str_append(str, log_prefix);
 }
 
-static void log_fd_flush_stop(struct ioloop *ioloop)
-{
-	io_loop_stop(ioloop);
-}
-
 static int log_fd_write(int fd, const unsigned char *data, size_t len)
 {
-	struct ioloop *ioloop;
-	struct io *io;
 	ssize_t ret;
 	unsigned int prev_signal_term_counter = signal_term_counter;
 	unsigned int terminal_eintr_count = 0;
+	const char *old_title = NULL;
+	int fd_orig_flags = -1;
+	bool failed = FALSE;
 
-	while ((ret = write(fd, data, len)) != (ssize_t)len) {
+	while (!failed &&
+	       (ret = write(fd, data, len)) != (ssize_t)len) {
 		if (ret > 0) {
 			/* some was written, continue.. */
 			data += ret;
@@ -128,17 +125,19 @@ static int log_fd_write(int fd, const unsigned char *data, size_t len)
 		if (ret == 0) {
 			/* out of disk space? */
 			errno = ENOSPC;
-			return -1;
+			failed = TRUE;
+			break;
 		}
 		switch (errno) {
 		case EAGAIN: {
-			/* wait until we can write more. this can happen at
-			   least when writing to terminal, even if fd is
-			   blocking. Internal logging fd is also now
-			   non-blocking, so we can show warnings about blocking
-			   on a log write. */
-			const char *title, *old_title =
-				t_strdup(process_title_get());
+			/* Update the process title to show that we're waiting
+			   on log writing. Switch to blocking writes and retry.
+			   When logging is finished, restore the process
+			   title. */
+			const char *title;
+
+			if (old_title == NULL)
+				old_title = t_strdup(process_title_get());
 
 			if (old_title == NULL)
 				title = "[blocking on log write]";
@@ -147,13 +146,22 @@ static int log_fd_write(int fd, const unsigned char *data, size_t len)
 							old_title);
 			process_title_set(title);
 
-			ioloop = io_loop_create();
-			io = io_add(fd, IO_WRITE, log_fd_flush_stop, ioloop);
-			io_loop_run(ioloop);
-			io_remove(&io);
-			io_loop_destroy(&ioloop);
-
-			process_title_set(old_title);
+			/* Now that the process title is updated, set fd to be
+			   blocking and retry. */
+			if (fd_orig_flags == -1) {
+				fd_orig_flags = fcntl(fd, F_GETFL, 0);
+				if (fd_orig_flags < 0)
+					i_fatal("fcntl(%d, F_GETFL) failed: %m", fd);
+
+				if ((fd_orig_flags & O_NONBLOCK) == 0) {
+					/* EAGAIN on a blocking fd? This is a
+					   bit weird, but just continue. */
+				} else {
+					int flags = fd_orig_flags | O_NONBLOCK;
+					if (fcntl(fd, F_SETFL, flags) < 0)
+						i_fatal("fcntl(%d, F_SETFL) failed: %m", fd);
+				}
+			}
 			break;
 		}
 		case EINTR:
@@ -165,15 +173,23 @@ static int log_fd_write(int fd, const unsigned char *data, size_t len)
 			} else {
 				/* received two terminal signals.
 				   someone wants us dead. */
-				return -1;
+				failed = TRUE;
+				break;
 			}
 			break;
 		default:
-			return -1;
+			failed = TRUE;
+			break;
 		}
 		prev_signal_term_counter = signal_term_counter;
 	}
-	return 0;
+	if (fd_orig_flags != -1) {
+		if (fcntl(fd, F_SETFL, fd_orig_flags) < 0)
+			i_fatal("fcntl(%d, F_SETFL) failed: %m", fd);
+	}
+	if (old_title != NULL)
+		process_title_set(old_title);
+	return failed ? -1 : 0;
 }
 
 static int ATTR_FORMAT(3, 0)