gitk: Allow users to view diffs in external diff viewer

This allows gitk to run an external diff viewer such as meld.

Right-click on a file in the file list view gives "External diff"
popup menu entry, which launches the selected external diff tool.
The menu entry is only active in "Patch" mode, not in "Tree" mode.

The program to run to display the diff is configurable through
Edit/Preference/External diff tool.  The program is run with two
arguments, being the names of files containing the two versions to
diff.  Gitk will create temporary directories called
.gitk-tmp.<pid>/<n> to place these files in, and remove them when
it's finished.

If the file doesn't exist in one or other revision, gitk will supply
/dev/null as the name of the file on that side of the diff.  This may
need to be adjusted for Windows or MacOS.

[paulus@samba.org - cleaned up and rewrote some parts of the patch.]

Signed-off-by: Thomas Arcila <thomas.arcila@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

gitk

@@ -1088,6 +1088,8 @@ proc makewindow {} {
 	-command {flist_hl 0}
     $flist_menu add command -label [mc "Highlight this only"] \
 	-command {flist_hl 1}
+    $flist_menu add command -label [mc "External diff"] \
+        -command {external_diff}
 }
 
 # Windows sends all mouse wheel events to the current focused window, not
@@ -1192,7 +1194,7 @@ proc savestuff {w} {
     global viewname viewfiles viewargs viewargscmd viewperm nextviewnum
     global cmitmode wrapcomment datetimeformat limitdiffs
     global colors bgcolor fgcolor diffcolors diffcontext selectbgcolor
-    global autoselect
+    global autoselect extdifftool
 
     if {$stuffsaved} return
     if {![winfo viewable .]} return
@@ -1218,6 +1220,7 @@ proc savestuff {w} {
 	puts $f [list set diffcolors $diffcolors]
 	puts $f [list set diffcontext $diffcontext]
 	puts $f [list set selectbgcolor $selectbgcolor]
+	puts $f [list set extdifftool $extdifftool]
 
 	puts $f "set geometry(main) [wm geometry .]"
 	puts $f "set geometry(topwidth) [winfo width .tf]"
@@ -1768,6 +1771,12 @@ proc pop_flist_menu {w X Y x y} {
 	set e [lindex $treediffs($diffids) [expr {$l-2}]]
     }
     set flist_menu_file $e
+    set xdiffstate "normal"
+    if {$cmitmode eq "tree"} {
+	set xdiffstate "disabled"
+    }
+    # Disable "External diff" item in tree mode
+    $flist_menu entryconf 2 -state $xdiffstate
     tk_popup $flist_menu $X $Y
 }
 
@@ -1783,6 +1792,113 @@ proc flist_hl {only} {
     set gdttype [mc "touching paths:"]
 }
 
+proc save_file_from_commit {filename output what} {
+    global nullfile
+
+    if {[catch {exec git show $filename -- > $output} err]} {
+	if {[string match "fatal: bad revision *" $err]} {
+	    return $nullfile
+	}
+	error_popup "Error getting \"$filename\" from $what: $err"
+	return {}
+    }
+    return $output
+}
+
+proc external_diff_get_one_file {diffid filename diffdir} {
+    global nullid nullid2 nullfile
+    global gitdir
+
+    if {$diffid == $nullid} {
+        set difffile [file join [file dirname $gitdir] $filename]
+	if {[file exists $difffile]} {
+	    return $difffile
+	}
+	return $nullfile
+    }
+    if {$diffid == $nullid2} {
+        set difffile [file join $diffdir "\[index\] [file tail $filename]"]
+        return [save_file_from_commit :$filename $difffile index]
+    }
+    set difffile [file join $diffdir "\[$diffid\] [file tail $filename]"]
+    return [save_file_from_commit $diffid:$filename $difffile \
+	       "revision $diffid"]
+}
+
+proc external_diff {} {
+    global gitktmpdir nullid nullid2
+    global flist_menu_file
+    global diffids
+    global diffnum
+    global gitdir extdifftool
+
+    if {[llength $diffids] == 1} {
+        # no reference commit given
+        set diffidto [lindex $diffids 0]
+        if {$diffidto eq $nullid} {
+            # diffing working copy with index
+            set diffidfrom $nullid2
+        } elseif {$diffidto eq $nullid2} {
+            # diffing index with HEAD
+            set diffidfrom "HEAD"
+        } else {
+            # use first parent commit
+            global parentlist selectedline
+            set diffidfrom [lindex $parentlist $selectedline 0]
+        }
+    } else {
+        set diffidfrom [lindex $diffids 0]
+        set diffidto [lindex $diffids 1]
+    }
+
+    # make sure that several diffs wont collide
+    if {![info exists gitktmpdir]} {
+	set gitktmpdir [file join [file dirname $gitdir] \
+			    [format ".gitk-tmp.%s" [pid]]]
+	if {[catch {file mkdir $gitktmpdir} err]} {
+	    error_popup "Error creating temporary directory $gitktmpdir: $err"
+	    unset gitktmpdir
+	    return
+	}
+	set diffnum 0
+    }
+    incr diffnum
+    set diffdir [file join $gitktmpdir $diffnum]
+    if {[catch {file mkdir $diffdir} err]} {
+	error_popup "Error creating temporary directory $diffdir: $err"
+	return
+    }
+
+    # gather files to diff
+    set difffromfile [external_diff_get_one_file $diffidfrom $flist_menu_file $diffdir]
+    set difftofile [external_diff_get_one_file $diffidto $flist_menu_file $diffdir]
+
+    if {$difffromfile ne {} && $difftofile ne {}} {
+        set cmd [concat | [shellsplit $extdifftool] \
+		     [list $difffromfile $difftofile]]
+        if {[catch {set fl [open $cmd r]} err]} {
+            file delete -force $diffdir
+            error_popup [mc "$extdifftool: command failed: $err"]
+        } else {
+            fconfigure $fl -blocking 0
+            filerun $fl [list delete_at_eof $fl $diffdir]
+        }
+    }
+}
+
+# delete $dir when we see eof on $f (presumably because the child has exited)
+proc delete_at_eof {f dir} {
+    while {[gets $f line] >= 0} {}
+    if {[eof $f]} {
+	if {[catch {close $f} err]} {
+	    error_popup "External diff viewer failed: $err"
+	}
+	file delete -force $dir
+	return 0
+    }
+    return 1
+}
+
 # Functions for adding and removing shell-type quoting
 
 proc shellquote {str} {
@@ -7881,9 +7997,15 @@ proc showtag {tag isnew} {
 
 proc doquit {} {
     global stopped
+    global gitktmpdir
+
     set stopped 100
     savestuff .
     destroy .
+
+    if {[info exists gitktmpdir]} {
+	catch {file delete -force $gitktmpdir}
+    }
 }
 
 proc mkfontdisp {font top which} {
@@ -8012,7 +8134,7 @@ proc doprefs {} {
     global maxwidth maxgraphpct
     global oldprefs prefstop showneartags showlocalchanges
     global bgcolor fgcolor ctext diffcolors selectbgcolor
-    global tabstop limitdiffs autoselect
+    global tabstop limitdiffs autoselect extdifftool
 
     set top .gitkprefs
     set prefstop $top
@@ -8064,6 +8186,15 @@ proc doprefs {} {
     pack $top.ldiff.b $top.ldiff.l -side left
     grid x $top.ldiff -sticky w
 
+    entry $top.extdifft -textvariable extdifftool
+    frame $top.extdifff
+    label $top.extdifff.l -text [mc "External diff tool" ] -font optionfont \
+	-padx 10
+    button $top.extdifff.b -text [mc "Choose..."] -font optionfont \
+	-command choose_extdiff
+    pack $top.extdifff.l $top.extdifff.b -side left
+    grid x $top.extdifff $top.extdifft -sticky w
+
     label $top.cdisp -text [mc "Colors: press to choose"]
     grid $top.cdisp - -sticky w -pady 10
     label $top.bg -padx 40 -relief sunk -background $bgcolor
@@ -8111,6 +8242,15 @@ proc doprefs {} {
     bind $top <Visibility> "focus $top.buts.ok"
 }
 
+proc choose_extdiff {} {
+    global extdifftool
+
+    set prog [tk_getOpenFile -title "External diff tool" -multiple false]
+    if {$prog ne {}} {
+	set extdifftool $prog
+    }
+}
+
 proc choosecolor {v vi w x cmd} {
     global $v
 
@@ -8539,6 +8679,8 @@ set limitdiffs 1
 set datetimeformat "%Y-%m-%d %H:%M:%S"
 set autoselect 1
 
+set extdifftool "meld"
+
 set colors {green red blue magenta darkgrey brown orange}
 set bgcolor white
 set fgcolor black
@@ -8685,6 +8827,7 @@ if {$mergeonly} {
 
 set nullid "0000000000000000000000000000000000000000"
 set nullid2 "0000000000000000000000000000000000000001"
+set nullfile "/dev/null"
 
 set have_tk85 [expr {[package vcompare $tk_version "8.5"] >= 0}]