perf tools: Add gzip_is_compressed function

Add implementation of the is_compressed callback for gzip.

Signed-off-by: Jiri Olsa <[e-mail]>
Cc: Alexander Shishkin <[e-mail]>
Cc: David Ahern <[e-mail]>
Cc: Michael Petlan <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/util/zlib.c

···
 #include <sys/mman.h>
 #include <zlib.h>
 #include <linux/compiler.h>
+#include <unistd.h>
 
 #include "util/compress.h"
 #include "util/util.h"
···
 	return ret == Z_STREAM_END ? 0 : -1;
 }
 
-bool gzip_is_compressed(const char *input __maybe_unused)
+bool gzip_is_compressed(const char *input)
 {
-	return true;
+	int fd = open(input, O_RDONLY);
+	const uint8_t magic[2] = { 0x1f, 0x8b };
+	char buf[2] = { 0 };
+	ssize_t rc;
+
+	if (fd < 0)
+		return -1;
+
+	rc = read(fd, buf, sizeof(buf));
+	close(fd);
+	return rc == sizeof(buf) ?
+	       memcmp(buf, magic, sizeof(buf)) == 0 : false;
 }