unsigned int word;
/*
- * Is it a zlib-compressed buffer? If so, the first byte
- * must be 0x78 (15-bit window size, deflated), and the
- * first 16-bit word is evenly divisible by 31. If so,
- * we are looking at the official format, not the experimental
- * one.
+ * We must determine if the buffer contains the standard
+ * zlib-deflated stream or the experimental format based
+ * on the in-pack object format. Compare the header byte
+ * for each format:
+ *
+ * RFC1950 zlib w/ deflate : 0www1000 : 0 <= www <= 7
+ * Experimental pack-based : Stttssss : ttt = 1,2,3,4
+ *
+ * If bit 7 is clear and bits 0-3 equal 8, the buffer MUST be
+ * in standard loose-object format, UNLESS it is a Git-pack
+ * format object *exactly* 8 bytes in size when inflated.
+ *
+ * However, RFC1950 also specifies that the 1st 16-bit word
+ * must be divisible by 31 - this checksum tells us our buffer
+ * is in the standard format, giving a false positive only if
+ * the 1st word of the Git-pack format object happens to be
+ * divisible by 31, ie:
+ * ((byte0 * 256) + byte1) % 31 = 0
+ * => 0ttt10000www1000 % 31 = 0
+ *
+ * As it happens, this case can only arise for www=3 & ttt=1
+ * - ie, a Commit object, which would have to be 8 bytes in
+ * size. As no Commit can be that small, we find that the
+ * combination of these two criteria (bitmask & checksum)
+ * can always correctly determine the buffer format.
*/
word = (map[0] << 8) + map[1];
- if (map[0] == 0x78 && !(word % 31))
+ if ((map[0] & 0x8F) == 0x08 && !(word % 31))
return 0;
else
return 1;
--- /dev/null
+#!/bin/sh
+#
+# Copyright (c) 2011 Roberto Tyley
+#
+
+test_description='Correctly identify and parse loose object headers
+
+There are two file formats for loose objects - the original standard
+format, and the experimental format introduced with Git v1.4.3, later
+deprecated with v1.5.3. Although Git no longer writes the
+experimental format, objects in both formats must be read, with the
+format for a given file being determined by the header.
+
+Detecting file format based on header is not entirely trivial, not
+least because the first byte of a zlib-deflated stream will vary
+depending on how much memory was allocated for the deflation window
+buffer when the object was written out (for example 4KB on Android,
+rather that 32KB on a normal PC).
+
+The loose objects used as test vectors have been generated with the
+following Git versions:
+
+standard format: Git v1.7.4.1
+experimental format: Git v1.4.3 (legacyheaders=false)
+standard format, deflated with 4KB window size: Agit/JGit on Android
+'
+
+. ./test-lib.sh
+
+assert_blob_equals() {
+ printf "%s" "$2" >expected &&
+ git cat-file -p "$1" >actual &&
+ test_cmp expected actual
+}
+
+test_expect_success setup '
+ cp -R "$TEST_DIRECTORY/t1013/objects" .git/
+ git --version
+'
+
+test_expect_success 'read standard-format loose objects' '
+ git cat-file tag 8d4e360d6c70fbd72411991c02a09c442cf7a9fa &&
+ git cat-file commit 6baee0540ea990d9761a3eb9ab183003a71c3696 &&
+ git ls-tree 7a37b887a73791d12d26c0d3e39568a8fb0fa6e8 &&
+ assert_blob_equals "257cc5642cb1a054f08cc83f2d943e56fd3ebe99" "foo$LF"
+'
+
+test_expect_success 'read experimental-format loose objects' '
+ git cat-file tag 76e7fa9941f4d5f97f64fea65a2cba436bc79cbb &&
+ git cat-file commit 7875c6237d3fcdd0ac2f0decc7d3fa6a50b66c09 &&
+ git ls-tree 95b1625de3ba8b2214d1e0d0591138aea733f64f &&
+ assert_blob_equals "2e65efe2a145dda7ee51d1741299f848e5bf752e" "a" &&
+ assert_blob_equals "9ae9e86b7bd6cb1472d9373702d8249973da0832" "ab" &&
+ assert_blob_equals "85df50785d62d3b05ab03d9cbf7e4a0b49449730" "abcd" &&
+ assert_blob_equals "1656f9233d999f61ef23ef390b9c71d75399f435" "abcdefgh" &&
+ assert_blob_equals "1e72a6b2c4a577ab0338860fa9fe87f761fc9bbd" "abcdefghi" &&
+ assert_blob_equals "70e6a83d8dcb26fc8bc0cf702e2ddeb6adca18fd" "abcdefghijklmnop" &&
+ assert_blob_equals "bd15045f6ce8ff75747562173640456a394412c8" "abcdefghijklmnopqrstuvwx"
+'
+
+test_expect_success 'read standard-format objects deflated with smaller window buffer' '
+ git cat-file tag f816d5255855ac160652ee5253b06cd8ee14165a &&
+ git cat-file tag 149cedb5c46929d18e0f118e9fa31927487af3b6
+'
+
+test_done
projects / git.git / commitdiff