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Did you wish that you could refer to the future commit by sha1? Now you can! 9 | 10 | ### Demo 11 | ``` 12 | $ git commit --allow-empty -m 'I am the parent of ${CHILD_SHA1}' 13 | [master 8207324] I am the parent of ${CHILD_SHA1} 14 | $ git commit --allow-empty -m 'I am the child' 15 | [master 1c4f972] I am the child 16 | $ git log --oneline HEAD~2..HEAD 17 | 1c4f972 I am the child 18 | 8207324 I am the parent of ${CHILD_SHA1} 19 | $ refer-to-child --parent=HEAD~1 --child=HEAD --prefix-length=6 20 | $ git log --oneline HEAD~2..HEAD 21 | 9428c8c I am the child 22 | cdd3ab5 I am the parent of 9428c8 23 | ``` 24 | 25 | ###How do I use it? 26 | Write `${CHILD_SHA1}` in your commit message whenever you want to refer to your child commit. Then after both the parent and the child are commited, run `refer-to-child` to update both commits. 27 | 28 | ###How does it work? 29 | It simply iterates through sha1 prefixes, trying each one in the parent commit message and seeing if the (rebased) child ends up with a matching sha1. Once it finds a match, it creates the new commits and updates HEAD to point to them. 30 | 31 | ###Notes 32 | * I don't know what will happen if the child is a merge commit. 33 | * If you use the `--extra-header` option then it will add an extra header to the raw commit, thus giving infinite tries to guess a good prefix. I don't know what git clients/tools this plays well with. -------------------------------------------------------------------------------- /refer-to-child.go: -------------------------------------------------------------------------------- 1 | package main 2 | 3 | import ( 4 | "bytes" 5 | "crypto/sha1" 6 | "flag" 7 | "fmt" 8 | "io" 9 | "os" 10 | "os/exec" 11 | "runtime" 12 | "strings" 13 | ) 14 | 15 | type result struct { 16 | parent, child []byte 17 | } 18 | 19 | func referToChildInParent(parent, child string, shaPrefixLength int, begin, end uint64, extraHeader string, ch chan *result) { 20 | // Find a place to insert the custom header, if we need to. 21 | idx := strings.Index(parent, "committer ") 22 | parentBeforeCommitterField := parent[:idx] 23 | parentAfterCommitterField := parent[idx:len(parent)] 24 | h := sha1.New() 25 | f := fmt.Sprintf("%%0%dx", shaPrefixLength) 26 | for ; begin < end; begin++ { 27 | var parentExtra string 28 | if extraHeader != "" { 29 | // Add custom field 30 | parentExtra = fmt.Sprintf("%s %x\n", extraHeader, begin) 31 | } 32 | // Generate a SHA1 prefix of the desired length 33 | childSha1Prefix := fmt.Sprintf(f, begin%(1<<(uint(shaPrefixLength)*4))) 34 | // Update parent commit message to use that prefix. 35 | newParentAfterCommitterField := strings.Replace( 36 | parentAfterCommitterField, "${CHILD_SHA1}", childSha1Prefix, -1) 37 | 38 | // Compute parent SHA1 39 | parentHeader := fmt.Sprintf("commit %d\x00", 40 | len(parentBeforeCommitterField)+len(parentExtra)+len(newParentAfterCommitterField)) 41 | h.Reset() 42 | io.WriteString(h, parentHeader) 43 | io.WriteString(h, parentBeforeCommitterField) 44 | io.WriteString(h, parentExtra) 45 | io.WriteString(h, newParentAfterCommitterField) 46 | parentSum := h.Sum(nil) 47 | parentSha1 := fmt.Sprintf("%40x", parentSum) 48 | 49 | // Update child's parent to new parent SHA1 50 | newChild := replaceParent(child, parentSha1) 51 | 52 | // Compute child SHA1 53 | h.Reset() 54 | io.WriteString(h, fmt.Sprintf("commit %d\x00", len(newChild))) 55 | io.WriteString(h, newChild) 56 | childSum := h.Sum(nil) 57 | childSha1 := fmt.Sprintf("%40x", childSum) 58 | 59 | // Check if child SHA1 matches the desired prefix 60 | if childSha1[:len(childSha1Prefix)] == childSha1Prefix { 61 | ch <- &result{ 62 | []byte(parentBeforeCommitterField + parentExtra + newParentAfterCommitterField), 63 | []byte(newChild)} 64 | } 65 | } 66 | ch <- nil 67 | } 68 | 69 | func replaceParent(commit, parentSha string) string { 70 | prefix := "parent " 71 | sha1Start := strings.Index(commit, prefix) + len(prefix) 72 | sha1End := sha1Start + strings.Index(commit[sha1Start:], "\n") 73 | return commit[:sha1Start] + parentSha + commit[sha1End:] 74 | } 75 | 76 | func hardReset(sha1 string) error { 77 | data, err := exec.Command("git", "reset", "--hard", sha1).CombinedOutput() 78 | if err != nil { 79 | return fmt.Errorf("'git reset' failed: %s", data) 80 | } 81 | return nil 82 | } 83 | 84 | func writeCommit(data []byte) (string, error) { 85 | cmd := exec.Command("git", "hash-object", "-t", "commit", "--stdin", "-w") 86 | cmd.Stdin = bytes.NewReader(data) 87 | data, err := cmd.CombinedOutput() 88 | if err != nil { 89 | return "", fmt.Errorf("'git hash-object' failed: %s", data) 90 | } 91 | return string(data), nil 92 | } 93 | 94 | func getCommit(sha1 string) ([]byte, error) { 95 | data, err := exec.Command("git", "show", "--pretty=raw", sha1).CombinedOutput() 96 | if err != nil { 97 | return nil, fmt.Errorf("'git show' failed: %s", data) 98 | } 99 | lines := bytes.Split(data, []byte("\n")) 100 | out := new(bytes.Buffer) 101 | for _, line := range lines { 102 | if bytes.HasPrefix(line, []byte("commit ")) { 103 | continue 104 | } 105 | if bytes.HasPrefix(line, []byte("diff ")) { 106 | // Reached end of commit message--remove trailing blank line 107 | out.Truncate(out.Len() - 1) 108 | break 109 | } 110 | if bytes.HasPrefix(line, []byte(" ")) { 111 | // Undo commit message indenting 112 | line = line[4:] 113 | } 114 | out.Write(line) 115 | out.WriteByte('\n') 116 | } 117 | return out.Bytes(), nil 118 | } 119 | 120 | func die(err string) { 121 | fmt.Fprintln(os.Stderr, err) 122 | os.Exit(1) 123 | } 124 | 125 | var parentSha1 = flag.String("parent", "", "sha1 of the parent commit") 126 | var childSha1 = flag.String("child", "", "sha1 of child commit") 127 | var sha1PrefixLength = flag.Int("prefix-length", 6, "length of SHA1 prefix to replace ${CHILD_SHA1} with") 128 | var parallelism = flag.Int("parallelism", 8, "number of parallel searches to conduct") 129 | var dryRun = flag.Bool("dry-run", false, "Don't actually write any objects or update HEAD; just try to find a good prefix") 130 | var extraHeaderName = flag.String("extra-header", "", "The name of an extra header to add to the parent (see README)") 131 | 132 | func main() { 133 | runtime.GOMAXPROCS(*parallelism) 134 | flag.Usage = func() { 135 | fmt.Fprintf(os.Stderr, "Usage of %s:\n", os.Args[0]) 136 | fmt.Fprintln(os.Stderr, 137 | "Given commit c and its parent p, update the current git repo's HEAD to"+ 138 | " point to c', which is the same as c except that it has parent p',"+ 139 | " which is the same as p except that ${CHILD_SHA1} is replaced with"+ 140 | " sha1(c') whenever it occurs in p's commit message.") 141 | flag.PrintDefaults() 142 | } 143 | flag.Parse() 144 | p, err := getCommit(*parentSha1) 145 | if err != nil { 146 | die(err.Error()) 147 | } 148 | c, err := getCommit(*childSha1) 149 | if err != nil { 150 | die(err.Error()) 151 | } 152 | ch := make(chan *result) 153 | start := uint64(0) 154 | inc := uint64(1) << (4 * uint(*sha1PrefixLength)) / uint64(*parallelism) 155 | var r *result 156 | for { 157 | for i := 0; i < *parallelism; i++ { 158 | go referToChildInParent(string(p), string(c), *sha1PrefixLength, start, start+inc, *extraHeaderName, ch) 159 | start += inc 160 | } 161 | for i := 0; i < *parallelism; i++ { 162 | r = <-ch 163 | if r != nil { 164 | break 165 | } 166 | } 167 | if r == nil { 168 | if *extraHeaderName == "" { 169 | die("Did not succeed just by updating the parent message." + 170 | " Specify -extra-header if you would like to try adding an" + 171 | " extra header to the parent to give us more text to play with.") 172 | } 173 | start += 1 << (4 * uint(*sha1PrefixLength)) 174 | } else { 175 | break 176 | } 177 | } 178 | 179 | if !*dryRun { 180 | // Reset to before parent 181 | if err = hardReset(*parentSha1 + "~1"); err != nil { 182 | die(err.Error()) 183 | } 184 | // Write new parent and child commits 185 | sha1, err := writeCommit(r.parent) 186 | if err != nil { 187 | die(err.Error()) 188 | } 189 | sha1, err = writeCommit(r.child) 190 | if err != nil { 191 | die(err.Error()) 192 | } 193 | 194 | // Set HEAD to new child 195 | if err = hardReset(strings.TrimSpace(sha1)); err != nil { 196 | die(err.Error()) 197 | } 198 | } 199 | 200 | os.Exit(0) 201 | } 202 | --------------------------------------------------------------------------------