config: support a configurable, and turn-off-able, pack.window

One use of go-git is to transfer git data from a non-standard git repo
(not stored in a file system, for example) to a "remote" backed by a
standard, local .git repo.

In this scenario, delta compression is not needed to reduce transfer
time over the "network", because there is no network. The underlying
storage layer has already taken care of the data tranfer, and sending
the objects to local .git storage doesn't require compression. So this
PR gives the user the option to turn off compression when it isn't
needed.

Of course, this results in a larger, uncompressed local .git repo, but
the user can then run git gc or git repack on that repo if they care
about the storage costs.

Turning the pack window to 0 on reduces total push time of a 36K repo
by 50 seconds (out of a pre-PR total of 3m26s).

1 files changed, 39 insertions, 0 deletions

diff --git a/config/config.go b/config/config.go
index 475045e..477eb35 100644
--- a/config/config.go
+++ b/config/config.go
@@ -6,6 +6,7 @@ import (
 	"errors"
 	"fmt"
 	"sort"
+	"strconv"
 
 	format "gopkg.in/src-d/go-git.v4/plumbing/format/config"
 )
@@ -40,6 +41,14 @@ type Config struct {
 		// Worktree is the path to the root of the working tree.
 		Worktree string
 	}
+
+	Pack struct {
+		// Window controls the size of the sliding window for delta
+		// compression.  The default is 10.  A value of 0 turns off
+		// delta compression entirely.
+		Window uint
+	}
+
 	// Remotes list of repository remotes, the key of the map is the name
 	// of the remote, should equal to RemoteConfig.Name.
 	Remotes map[string]*RemoteConfig
@@ -81,10 +90,14 @@ const (
 	remoteSection    = "remote"
 	submoduleSection = "submodule"
 	coreSection      = "core"
+	packSection      = "pack"
 	fetchKey         = "fetch"
 	urlKey           = "url"
 	bareKey          = "bare"
 	worktreeKey      = "worktree"
+	windowKey        = "window"
+
+	defaultPackWindow = uint(10)
 )
 
 // Unmarshal parses a git-config file and stores it.
@@ -98,6 +111,9 @@ func (c *Config) Unmarshal(b []byte) error {
 	}
 
 	c.unmarshalCore()
+	if err := c.unmarshalPack(); err != nil {
+		return err
+	}
 	c.unmarshalSubmodules()
 	return c.unmarshalRemotes()
 }
@@ -111,6 +127,21 @@ func (c *Config) unmarshalCore() {
 	c.Core.Worktree = s.Options.Get(worktreeKey)
 }
 
+func (c *Config) unmarshalPack() error {
+	s := c.Raw.Section(packSection)
+	window := s.Options.Get(windowKey)
+	if window == "" {
+		c.Pack.Window = defaultPackWindow
+	} else {
+		winUint, err := strconv.ParseUint(window, 10, 32)
+		if err != nil {
+			return err
+		}
+		c.Pack.Window = uint(winUint)
+	}
+	return nil
+}
+
 func (c *Config) unmarshalRemotes() error {
 	s := c.Raw.Section(remoteSection)
 	for _, sub := range s.Subsections {
@@ -138,6 +169,7 @@ func (c *Config) unmarshalSubmodules() {
 // Marshal returns Config encoded as a git-config file.
 func (c *Config) Marshal() ([]byte, error) {
 	c.marshalCore()
+	c.marshalPack()
 	c.marshalRemotes()
 	c.marshalSubmodules()
 
@@ -158,6 +190,13 @@ func (c *Config) marshalCore() {
 	}
 }
 
+func (c *Config) marshalPack() {
+	s := c.Raw.Section(packSection)
+	if c.Pack.Window != defaultPackWindow {
+		s.SetOption(windowKey, fmt.Sprintf("%d", c.Pack.Window))
+	}
+}
+
 func (c *Config) marshalRemotes() {
 	s := c.Raw.Section(remoteSection)
 	newSubsections := make(format.Subsections, 0, len(c.Remotes))