perf intel-pt: Remove first line of log dumped on error

Instead of printing "(first line may be sliced)", always remove the
first line of the debug log if the buffer has wrapped when dumping on
error.
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20220905073424.3971-7-adrian.hunter@intel.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/intel-pt-decoder/intel-pt-log.c

@@ -143,16 +143,39 @@ static FILE *log_buf__open(struct log_buf *b, FILE *backend, unsigned int sz)
 	return file;
 }
 
+static bool remove_first_line(const char **p, size_t *n)
+{
+	for (; *n && **p != '\n'; ++*p, --*n)
+		;
+	if (*n) {
+		*p += 1;
+		*n -= 1;
+		return true;
+	}
+	return false;
+}
+
+static void write_lines(const char *p, size_t n, FILE *fp, bool *remove_first)
+{
+	if (*remove_first)
+		*remove_first = !remove_first_line(&p, &n);
+	fwrite(p, n, 1, fp);
+}
+
 static void log_buf__dump(struct log_buf *b)
 {
+	bool remove_first = false;
+
 	if (!b->buf)
 		return;
 
-	fflush(f);
-	fprintf(b->backend, "Dumping debug log buffer (first line may be sliced)\n");
-	if (b->wrapped)
-		fwrite(b->buf + b->head, b->buf_sz - b->head, 1, b->backend);
-	fwrite(b->buf, b->head, 1, b->backend);
+	fflush(f); /* Could update b->head and b->wrapped */
+	fprintf(b->backend, "Dumping debug log buffer\n");
+	if (b->wrapped) {
+		remove_first = true;
+		write_lines(b->buf + b->head, b->buf_sz - b->head, b->backend, &remove_first);
+	}
+	write_lines(b->buf, b->head, b->backend, &remove_first);
 	fprintf(b->backend, "End of debug log buffer dump\n");
 
 	b->head = 0;