io_archive: Use SOURCE_DATE_EPOCH for meta files instead of current time

[TT: minor stylistic changes]

src/apk_defines.h

@@ -177,6 +177,8 @@ static inline uint32_t get_unaligned32(const void *ptr)
 
 typedef void (*apk_progress_cb)(void *cb_ctx, size_t);
 
+time_t apk_get_build_time(void);
+
 void *apk_array_resize(void *array, size_t new_size, size_t elem_size);
 
 #define APK_ARRAY(array_type_name, elem_type_name)			\

src/common.c

@@ -40,3 +40,18 @@ void *apk_array_resize(void *array, size_t new_size, size_t elem_size)
 
 	return tmp;
 }
+
+time_t apk_get_build_time(void)
+{
+	static int initialized = 0;
+	static time_t timestamp = 0;
+	char *source_date_epoch;
+
+	if (initialized) return timestamp;
+	source_date_epoch = getenv("SOURCE_DATE_EPOCH");
+	if (source_date_epoch && *source_date_epoch)
+		timestamp = strtoull(source_date_epoch, NULL, 10);
+	else	timestamp = time(NULL);
+	initialized = 1;
+	return timestamp;
+}

src/io_archive.c

@@ -291,7 +291,7 @@ int apk_tar_write_entry(struct apk_ostream *os, const struct apk_file_info *ae,
 		PUT_OCTAL(buf.uid, ae->uid);
 		PUT_OCTAL(buf.gid, ae->gid);
 		PUT_OCTAL(buf.mode, ae->mode & 07777);
-		PUT_OCTAL(buf.mtime, ae->mtime ?: time(NULL));
+		PUT_OCTAL(buf.mtime, ae->mtime ?: apk_get_build_time());
 
 		/* Checksum */
 		strcpy(buf.magic, "ustar  ");