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perf(gitdiff): optimize inline diff highlighting using cache (#38706)

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This pull request optimizes code diff highlighting in the PR Files
Changed view, which is a major CPU bottleneck and hot path in
production.

Previously, Gitea executed the expensive `DiffMatchPatch` algorithm
twice for every matching deleted/added line pair (once for the deleted
line, and once for the added line). We now run the diff algorithm once
and cache the resulting `DiffInline` on the `DiffLine` struct itself,
allowing the second line to render via a 0-cost cache lookup.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>
This commit is contained in:
Shudhanshu Singh
2026-07-30 22:13:36 +05:30
committed by GitHub
parent 11d0ed699b
commit d87d26d735
11 changed files with 186 additions and 157 deletions
Download Patch File Download Diff File Expand all files Collapse all files
services/gitdiff/highlightdiff_test.go
+58 -46
View File
@@ -10,31 +10,20 @@ import (
"testing"
"gitea.dev/modules/highlight"
"gitea.dev/modules/translation"
"github.com/alecthomas/chroma/v2"
"github.com/stretchr/testify/assert"
)
func BenchmarkHighlightDiff(b *testing.B) {
for b.Loop() {
// still fast enough: BenchmarkHighlightDiff-12 1000000 1027 ns/op
// TODO: the real bottleneck is that "diffLineWithHighlight" is called twice when rendering "added" and "removed" lines by the caller
// Ideally the caller should cache the diff result, and then use the diff result to render "added" and "removed" lines separately
hcd := newHighlightCodeDiff()
codeA := template.HTML(`x <span class="k">foo</span> y`)
codeB := template.HTML(`x <span class="k">bar</span> y`)
hcd.diffLineWithHighlight(DiffLineDel, codeA, codeB)
}
}
func TestDiffWithHighlight(t *testing.T) {
t.Run("DiffLineAddDel", func(t *testing.T) {
t.Run("WithDiffTags", func(t *testing.T) {
hcd := newHighlightCodeDiff()
codeA := template.HTML(`x <span class="k">foo</span> y`)
codeB := template.HTML(`x <span class="k">bar</span> y`)
outDel := hcd.diffLineWithHighlight(DiffLineDel, codeA, codeB)
outDel, outAdd := hcd.diffLineWithHighlight(codeA, codeB)
assert.Equal(t, `x <span class="removed-code"><span class="k">foo</span></span> y`, string(outDel))
outAdd := hcd.diffLineWithHighlight(DiffLineAdd, codeA, codeB)
assert.Equal(t, `x <span class="added-code"><span class="k">bar</span></span> y`, string(outAdd))
})
t.Run("NoRedundantTags", func(t *testing.T) {
@@ -43,9 +32,8 @@ func TestDiffWithHighlight(t *testing.T) {
hcd := newHighlightCodeDiff()
codeA := template.HTML("<span> </span> \t<span>foo</span> ")
codeB := template.HTML(" <span>bar</span> \n")
outDel := hcd.diffLineWithHighlight(DiffLineDel, codeA, codeB)
outDel, outAdd := hcd.diffLineWithHighlight(codeA, codeB)
assert.Equal(t, string(codeA), string(outDel))
outAdd := hcd.diffLineWithHighlight(DiffLineAdd, codeA, codeB)
assert.Equal(t, string(codeB), string(outAdd))
})
})
@@ -54,16 +42,14 @@ func TestDiffWithHighlight(t *testing.T) {
hcd := newHighlightCodeDiff()
codeA := template.HTML(` <span class="cm">this is a comment</span>`)
codeB := template.HTML(` <span class="cm">this is updated comment</span>`)
outDel := hcd.diffLineWithHighlight(DiffLineDel, codeA, codeB)
outDel, outAdd := hcd.diffLineWithHighlight(codeA, codeB)
assert.Equal(t, ` <span class="cm">this is <span class="removed-code">a</span> comment</span>`, string(outDel))
outAdd := hcd.diffLineWithHighlight(DiffLineAdd, codeA, codeB)
assert.Equal(t, ` <span class="cm">this is <span class="added-code">updated</span> comment</span>`, string(outAdd))
codeA = `<span class="line"><span>line1</span></span>` + "\n" + `<span class="cl"><span>line2</span></span>`
codeB = `<span class="cl"><span>line1</span></span>` + "\n" + `<span class="line"><span>line!</span></span>`
outDel = hcd.diffLineWithHighlight(DiffLineDel, codeA, codeB)
outDel, outAdd = hcd.diffLineWithHighlight(codeA, codeB)
assert.Equal(t, `<span>line1</span>`+"\n"+`<span class="removed-code"><span>line2</span></span>`, string(outDel))
outAdd = hcd.diffLineWithHighlight(DiffLineAdd, codeA, codeB)
assert.Equal(t, `<span>line1</span>`+"\n"+`<span><span class="added-code">line!</span></span>`, string(outAdd))
})
@@ -79,12 +65,12 @@ func TestDiffWithHighlight(t *testing.T) {
oldCode, _, _ := highlight.RenderCodeSlowGuess("a.go", "Go", `xxx || yyy`)
newCode, _, _ := highlight.RenderCodeSlowGuess("a.go", "Go", `bot&xxx || bot&yyy`)
hcd := newHighlightCodeDiff()
out := hcd.diffLineWithHighlight(DiffLineAdd, oldCode, newCode)
_, add := hcd.diffLineWithHighlight(oldCode, newCode)
assert.Equal(t, strings.ReplaceAll(`
<span class="added-code"><span class="nx">bot</span></span><span class="o"><span class="added-code">&amp;</span></span>
<span class="nx">xxx</span><span class="w"> </span><span class="o">||</span><span class="w"> </span>
<span class="added-code"><span class="nx">bot</span></span><span class="o"><span class="added-code">&amp;</span></span>
<span class="nx">yyy</span>`, "\n", ""), string(out))
<span class="nx">yyy</span>`, "\n", ""), string(add))
})
forceTokenAsPlaceholder := func(hcd *highlightCodeDiff, r rune, token string) rune {
@@ -127,14 +113,14 @@ func TestDiffWithHighlight(t *testing.T) {
func TestDiffWithHighlightPlaceholder(t *testing.T) {
hcd := newHighlightCodeDiff()
output := hcd.diffLineWithHighlight(DiffLineDel, "a='\U00100000'", "a='\U0010FFFD''")
output, _ := hcd.diffLineWithHighlight("a='\U00100000'", "a='\U0010FFFD''")
assert.Empty(t, hcd.placeholderTokenMap[0x00100000])
assert.Empty(t, hcd.placeholderTokenMap[0x0010FFFD])
expected := fmt.Sprintf(`a='<span class="removed-code">%s</span>'`, "\U00100000")
assert.Equal(t, expected, string(output))
hcd = newHighlightCodeDiff()
output = hcd.diffLineWithHighlight(DiffLineAdd, "a='\U00100000'", "a='\U0010FFFD'")
_, output = hcd.diffLineWithHighlight("a='\U00100000'", "a='\U0010FFFD'")
expected = fmt.Sprintf(`a='<span class="added-code">%s</span>'`, "\U0010FFFD")
assert.Equal(t, expected, string(output))
}
@@ -143,32 +129,58 @@ func TestDiffWithHighlightPlaceholderExhausted(t *testing.T) {
hcd := newHighlightCodeDiff()
hcd.placeholderMaxCount = 0
placeHolderAmp := string(rune(0xFFFD))
output := hcd.diffLineWithHighlight(DiffLineDel, `<span class="k">&lt;</span>`, `<span class="k">&gt;</span>`)
assert.Equal(t, placeHolderAmp+"lt;", string(output))
output = hcd.diffLineWithHighlight(DiffLineAdd, `<span class="k">&lt;</span>`, `<span class="k">&gt;</span>`)
assert.Equal(t, placeHolderAmp+"gt;", string(output))
del, add := hcd.diffLineWithHighlight(`<span class="k">&lt;</span>`, `<span class="k">&gt;</span>`)
assert.Equal(t, placeHolderAmp+"lt;", string(del))
assert.Equal(t, placeHolderAmp+"gt;", string(add))
output = hcd.diffLineWithHighlight(DiffLineDel, `<span class="k">foo</span>`, `<span class="k">bar</span>`)
assert.Equal(t, "foo", string(output))
output = hcd.diffLineWithHighlight(DiffLineAdd, `<span class="k">foo</span>`, `<span class="k">bar</span>`)
assert.Equal(t, "bar", string(output))
del, add = hcd.diffLineWithHighlight(`<span class="k">foo</span>`, `<span class="k">bar</span>`)
assert.Equal(t, "foo", string(del))
assert.Equal(t, "bar", string(add))
}
func TestDiffWithHighlightTagMatch(t *testing.T) {
f := func(t *testing.T, lineType DiffLineType) {
totalOverflow := 0
for i := 0; ; i++ {
hcd := newHighlightCodeDiff()
hcd.placeholderMaxCount = i
output := string(hcd.diffLineWithHighlight(lineType, `<span class="k">&lt;</span>`, `<span class="k">&gt;</span>`))
totalOverflow += hcd.placeholderOverflowCount
assert.Equal(t, strings.Count(output, "<span"), strings.Count(output, "</span"))
if hcd.placeholderOverflowCount == 0 {
break
}
totalOverflow := 0
for i := 0; ; i++ {
hcd := newHighlightCodeDiff()
hcd.placeholderMaxCount = i
del, add := hcd.diffLineWithHighlight(`<span class="k">&lt;</span>`, `<span class="k">&gt;</span>`)
totalOverflow += hcd.placeholderOverflowCount
assert.Equal(t, strings.Count(string(del), "<span"), strings.Count(string(del), "</span"))
assert.Equal(t, strings.Count(string(add), "<span"), strings.Count(string(add), "</span"))
if hcd.placeholderOverflowCount == 0 {
break
}
assert.NotZero(t, totalOverflow)
}
t.Run("DiffLineAdd", func(t *testing.T) { f(t, DiffLineAdd) })
t.Run("DiffLineDel", func(t *testing.T) { f(t, DiffLineDel) })
assert.NotZero(t, totalOverflow)
}
func BenchmarkHighlightDiff(b *testing.B) {
// still fast enough: BenchmarkHighlightDiff-12 1000000 1027 ns/op
// HINT: CODE-HIGHLIGHT-PERFORMANCE: the real bottleneck is in the Chroma highlighter.
for b.Loop() {
hcd := newHighlightCodeDiff()
codeA := template.HTML(`x <span class="k">foo</span> y`)
codeB := template.HTML(`x <span class="k">bar</span> y`)
hcd.diffLineWithHighlight(codeA, codeB)
}
}
func BenchmarkGetDiffLineForRender(b *testing.B) {
diffSection := &DiffSection{
FileName: "test.go",
highlightLexer: &diffVarMutable[chroma.Lexer]{},
}
leftLine := &DiffLine{LeftIdx: 1, Content: `-x <span class="k">foo</span> y`}
rightLine := &DiffLine{RightIdx: 1, Content: `+x <span class="k">bar</span> y`}
locale := translation.MockLocale{}
b.ResetTimer()
// HINT: CODE-HIGHLIGHT-PERFORMANCE: the real bottleneck is in the Chroma highlighter.
for b.Loop() {
// Clear cache only at the start of rendering the pair
leftLine.cachedDiffInline = nil
rightLine.cachedDiffInline = nil
_ = diffSection.getDiffLineForRender(DiffLineDel, leftLine, rightLine, locale)
_ = diffSection.getDiffLineForRender(DiffLineAdd, leftLine, rightLine, locale)
}
}