Add Plan 9 support

Not all the tests pass yet but this makes go-git usable on Plan 9.
Please merge this after https://github.com/src-d/go-billy/pull/78.

Fixes #756

Signed-off-by: Fazlul Shahriar <fshahriar@gmail.com>

3 files changed, 80 insertions, 0 deletions

diff --git a/go.sum b/go.sum
index 65551c1..21a89fb 100644
--- a/go.sum
+++ b/go.sum
@@ -18,6 +18,7 @@ github.com/gliderlabs/ssh v0.2.2 h1:6zsha5zo/TWhRhwqCD3+EarCAgZ2yN28ipRnGPnwkI0=
 github.com/gliderlabs/ssh v0.2.2/go.mod h1:U7qILu1NlMHj9FlMhZLlkCdDnU1DBEAqr0aevW3Awn0=
 github.com/google/go-cmp v0.2.0 h1:+dTQ8DZQJz0Mb/HjFlkptS1FeQ4cWSnN941F8aEG4SQ=
 github.com/google/go-cmp v0.2.0/go.mod h1:oXzfMopK8JAjlY9xF4vHSVASa0yLyX7SntLO5aqRK0M=
+github.com/google/go-cmp v0.3.0 h1:crn/baboCvb5fXaQ0IJ1SGTsTVrWpDsCWC8EGETZijY=
 github.com/google/go-cmp v0.3.0/go.mod h1:8QqcDgzrUqlUb/G2PQTWiueGozuR1884gddMywk6iLU=
 github.com/jbenet/go-context v0.0.0-20150711004518-d14ea06fba99 h1:BQSFePA1RWJOlocH6Fxy8MmwDt+yVQYULKfN0RoTN8A=
 github.com/jbenet/go-context v0.0.0-20150711004518-d14ea06fba99/go.mod h1:1lJo3i6rXxKeerYnT8Nvf0QmHCRC1n8sfWVwXF2Frvo=
@@ -77,6 +78,7 @@ golang.org/x/sys v0.0.0-20190726091711-fc99dfbffb4e h1:D5TXcfTk7xF7hvieo4QErS3qq
 golang.org/x/sys v0.0.0-20190726091711-fc99dfbffb4e/go.mod h1:h1NjWce9XRLGQEsW7wpKNCjG9DtNlClVuFLEZdDNbEs=
 golang.org/x/text v0.3.0 h1:g61tztE5qeGQ89tm6NTjjM9VPIm088od1l6aSorWRWg=
 golang.org/x/text v0.3.0/go.mod h1:NqM8EUOU14njkJ3fqMW+pc6Ldnwhi/IjpwHt7yyuwOQ=
+golang.org/x/text v0.3.2 h1:tW2bmiBqwgJj/UpqtC8EpXEZVYOwU0yG4iWbprSVAcs=
 golang.org/x/text v0.3.2/go.mod h1:bEr9sfX3Q8Zfm5fL9x+3itogRgK3+ptLWKqgva+5dAk=
 golang.org/x/tools v0.0.0-20180917221912-90fa682c2a6e/go.mod h1:n7NCudcB/nEzxVGmLbDWY5pfWTLqBcC2KZ6jyYvM4mQ=
 golang.org/x/tools v0.0.0-20190729092621-ff9f1409240a/go.mod h1:jcCCGcm9btYwXyDqrUWc6MKQKKGJCWEQ3AfLSRIbEuI=
diff --git a/repository_plan9_test.go b/repository_plan9_test.go
new file mode 100644
index 0000000..00ebeed
--- /dev/null
+++ b/repository_plan9_test.go
@@ -0,0 +1,47 @@
+package git
+
+import (
+	"fmt"
+	"strings"
+)
+
+// preReceiveHook returns the bytes of a pre-receive hook script
+// that prints m before exiting successfully
+func preReceiveHook(m string) []byte {
+	return []byte(fmt.Sprintf("#!/bin/rc\necho -n %s\n", quote(m)))
+}
+
+const quoteChar = '\''
+
+func needsQuote(s string) bool {
+	for i := 0; i < len(s); i++ {
+		c := s[i]
+		if c == quoteChar || c <= ' ' { // quote, blanks, or control characters
+			return true
+		}
+	}
+	return false
+}
+
+// Quote adds single quotes to s in the style of rc(1) if they are needed.
+// The behaviour should be identical to Plan 9's quote(3).
+func quote(s string) string {
+	if s == "" {
+		return "''"
+	}
+	if !needsQuote(s) {
+		return s
+	}
+	var b strings.Builder
+	b.Grow(10 + len(s)) // Enough room for few quotes
+	b.WriteByte(quoteChar)
+	for i := 0; i < len(s); i++ {
+		c := s[i]
+		if c == quoteChar {
+			b.WriteByte(quoteChar)
+		}
+		b.WriteByte(c)
+	}
+	b.WriteByte(quoteChar)
+	return b.String()
+}
diff --git a/worktree_plan9.go b/worktree_plan9.go
new file mode 100644
index 0000000..16d3915
--- /dev/null
+++ b/worktree_plan9.go
@@ -0,0 +1,31 @@
+package git
+
+import (
+	"syscall"
+	"time"
+
+	"gopkg.in/src-d/go-git.v4/plumbing/format/index"
+)
+
+func init() {
+	fillSystemInfo = func(e *index.Entry, sys interface{}) {
+		if os, ok := sys.(*syscall.Dir); ok {
+			// Plan 9 doesn't have a CreatedAt field.
+			e.CreatedAt = time.Unix(int64(os.Mtime), 0)
+
+			e.Dev = uint32(os.Dev)
+
+			// Plan 9 has no Inode.
+			// ext2srv(4) appears to store Inode in Qid.Path.
+			e.Inode = uint32(os.Qid.Path)
+
+			// Plan 9 has string UID/GID
+			e.GID = 0
+			e.UID = 0
+		}
+	}
+}
+
+func isSymlinkWindowsNonAdmin(err error) bool {
+	return true
+}