Avoid duplicate fatal stderr output in GUI mode

cmdline/snapraid.c

@@ -178,16 +178,18 @@ void log_open(const char* file)
 
 void log_close(const char* file)
 {
-	if (stdlog != stdout && stdlog != stderr && stdlog != 0) {
-		if (fclose(stdlog) != 0) {
+	FILE* log = stdlog;
+
+	/* fclose() invalidates the stream even on error, so fatal reporting must use stderr. */
+	stdlog = 0;
+	if (log != stdout && log != stderr && log != 0) {
+		if (fclose(log) != 0) {
 			/* LCOV_EXCL_START */
 			log_fatal(errno, "Error closing the log file '%s'. %s.\n", file, strerror(errno));
 			exit(EXIT_FAILURE);
 			/* LCOV_EXCL_STOP */
 		}
 	}
-
-	stdlog = 0;
 }
 
 /****************************************************************************/
@@ -1065,6 +1067,8 @@ int snapraid_main(int argc, char* argv[])
 		}
 	}
 
+	msg_fatal_stderr = !opt.gui;
+
 	os_init(OS_INIT_OPT_AVOID_SLEEP | (opt.force_scan_winfind ? OS_INIT_OPT_WINFIND : 0));
 	app_init();
 	raid_init();

cmdline/support.c

@@ -293,6 +293,7 @@ void log_error_errno(int err, const char* name)
 /* print */
 
 int msg_level = 0;
+int msg_fatal_stderr = 1;
 FILE* stdlog = 0;
 int msg_line_prev = 0; /**< Previous line width on stdout */
 int msg_line_curr = 0; /**< Current line width on stdout */
@@ -374,34 +375,41 @@ static void msg(FILE* out, const char* format, ...)
 
 void vlog_fatal(int err, const char* format, va_list ap, const char* post)
 {
+	int log_failed = 0;
+
 	lock_msg();
 
 	if (stdlog) {
 		if (is_hw(err))
-			fprintf(stdlog, "msg:fatal_hardware: ");
+			log_failed = fprintf(stdlog, "msg:fatal_hardware: ") < 0;
 		else
-			fprintf(stdlog, "msg:fatal: ");
+			log_failed = fprintf(stdlog, "msg:fatal: ") < 0;
 
 		va_list ap_copy;
 		va_copy(ap_copy, ap);
-		vfprintf(stdlog, format, ap_copy);
+		if (vfprintf(stdlog, format, ap_copy) < 0)
+			log_failed = 1;
 		va_end(ap_copy);
 
-		if (post)
-			fprintf(stdlog, "%s", post);
+		if (post && fprintf(stdlog, "%s", post) < 0)
+			log_failed = 1;
 
-		fflush(stdlog);
+		if (fflush(stdlog) != 0)
+			log_failed = 1;
 	}
 
-	va_list ap_copy2;
-	va_copy(ap_copy2, ap);
-	vmsg(stderr, format, ap_copy2);
-	va_end(ap_copy2);
+	/* A failed tagged write must retain stderr as the only usable fatal diagnostic. */
+	if (!stdlog || log_failed || msg_fatal_stderr) {
+		va_list ap_copy2;
+		va_copy(ap_copy2, ap);
+		vmsg(stderr, format, ap_copy2);
+		va_end(ap_copy2);
 
-	if (post)
-		msg(stderr, "%s", post);
+		if (post)
+			msg(stderr, "%s", post);
 
-	fflush(stderr);
+		fflush(stderr);
+	}
 
 	unlock_msg();
 }

cmdline/support.h

@@ -92,7 +92,8 @@ typedef void log_ptr(int err, const char* format, ...) __attribute__((format(att
  *
  * Messages printed before an early termination.
  *
- * These messages go in the log file and in stderr unconditionally.
+ * These messages go in the log file and in stderr, except in GUI mode
+ * when the tagged log message is emitted successfully.
  */
 void vlog_fatal(int err, const char* format, va_list ap, const char* post);
 void log_fatal(int err, const char* format, ...) __attribute__((format(attribute_printf, 2, 3)));
@@ -176,6 +177,11 @@ typedef void msg_ptr(const char* format, ...) __attribute__((format(attribute_pr
  */
 extern int msg_level;
 
+/**
+ * Emit fatal messages on stderr also when a log is configured.
+ */
+extern int msg_fatal_stderr;
+
 /**
  * State messages.
  *

doc/snapraid_log.1

@@ -70,6 +70,11 @@ is escaped to \\\\.
 Some tags are output only when the \`\-\-gui\` or \`\-\-gui\-verbose\` option
 is used. They provide extra verbose state and runtime progress only
 useful for a GUI interface.
+In GUI mode, the structured log is intended to be consumed by the GUI.
+Fatal errors are therefore not duplicated to standard error when they
+are successfully written to the log. If the log is unavailable or
+writing it fails, the fatal error is printed to standard error as a
+fallback.
 .PP
 Integer values are marked as \`uint\` when they are unsigned and as \`int\`
 when they are signed. However, for practical purposes you can treat

doc/snapraid_log.d

@@ -50,6 +50,11 @@ Description
 	Some tags are output only when the `--gui` or `--gui-verbose` option
 	is used. They provide extra verbose state and runtime progress only
 	useful for a GUI interface.
+	In GUI mode, the structured log is intended to be consumed by the GUI.
+	Fatal errors are therefore not duplicated to standard error when they
+	are successfully written to the log. If the log is unavailable or
+	writing it fails, the fatal error is printed to standard error as a
+	fallback.
 
 	Integer values are marked as `uint` when they are unsigned and as `int`
 	when they are signed. However, for practical purposes you can treat

doc/snapraid_log.txt

@@ -54,6 +54,11 @@ The following characters are escaped into sequences:
 Some tags are output only when the `--gui` or `--gui-verbose` option
 is used. They provide extra verbose state and runtime progress only
 useful for a GUI interface.
+In GUI mode, the structured log is intended to be consumed by the GUI.
+Fatal errors are therefore not duplicated to standard error when they
+are successfully written to the log. If the log is unavailable or
+writing it fails, the fatal error is printed to standard error as a
+fallback.
 
 Integer values are marked as `uint` when they are unsigned and as `int`
 when they are signed. However, for practical purposes you can treat