Fix RGBFIX --output with a pre-existing or same-as-input file (#2145)

src/fix/fix.cpp

@@ -381,6 +381,24 @@ static void
 		if (options.padValue == UNSPECIFIED) {
 			rom0Len = headerSize;
 		}
+	} else if (output != STDOUT_FILENO) {
+		// Truncate a pre-existing output file before writing to it
+		if (ftruncate(output, 0) != 0) {
+			// LCOV_EXCL_START
+			int errnum = errno;
+			if (errnum == EINVAL) {
+				// We do not support FIFOs or symlinks
+				if (struct stat stat; fstat(output, &stat) == 0 && !S_ISREG(stat.st_mode)) {
+					error(
+					    "\"%s\" is not a regular file, and thus cannot be modified in-place", name
+					);
+					return;
+				}
+			}
+			error("Failed to truncate \"%s\": %s", name, strerror(errnum));
+			return;
+			// LCOV_EXCL_STOP
+		}
 	}
 	writeLen = writeBytes(output, rom0, rom0Len);
 
@@ -498,8 +516,7 @@ bool fix_ProcessFile(char const *name, char const *outputName) {
 		error("Failed to open \"%s\" for reading+writing: %s", name, strerror(errno));
 	} else {
 		Defer closeInput{[&] { xclose(input); }};
-		struct stat stat;
-		if (fstat(input, &stat) == -1) {
+		if (struct stat stat; fstat(input, &stat) == -1) {
 			error("Failed to stat \"%s\": %s", name, strerror(errno)); // LCOV_EXCL_LINE
 		} else if (!S_ISREG(stat.st_mode)) { // We do not support FIFOs or symlinks
 			// LCOV_EXCL_START