dotenv: fix an unintended buffer copy

before this change: the test allocates ~10MB
after this change: the test allocates ~300KB

fixes docker/compose#10951

Signed-off-by: Nick Santos <nick.santos@docker.com>

Author: nicks

dotenv/parser.go

@@ -98,7 +98,11 @@ func (p *parser) locateKeyName(src string) (string, string, bool, error) {
 	var key string
 	var inherited bool
 	// trim "export" and space at beginning
-	src = strings.TrimLeftFunc(exportRegex.ReplaceAllString(src, ""), isSpace)
+	if exportRegex.MatchString(src) {
+		// we use a `strings.trim` to preserve the pointer to the same underlying memory.
+		// a regexp replace would copy the string.
+		src = strings.TrimLeftFunc(strings.TrimPrefix(src, "export"), isSpace)
+	}
 
 	// locate key name end and validate it in single loop
 	offset := 0

dotenv/parser_test.go

@@ -1,6 +1,9 @@
 package dotenv
 
 import (
+	"fmt"
+	"runtime"
+	"strings"
 	"testing"
 
 	"gotest.tools/v3/assert"
@@ -38,3 +41,24 @@ func TestParseVariable(t *testing.T) {
 	assert.Error(t, err, "line 1: unexpected character \"%\" in variable name \"%!(EXTRA string)=foo\"")
 
 }
+
+func TestMemoryExplosion(t *testing.T) {
+	p := newParser()
+	var startMemStats runtime.MemStats
+	var endMemStats runtime.MemStats
+	runtime.ReadMemStats(&startMemStats)
+
+	size := 1000
+	input := []string{}
+	for i := 0; i < size; i++ {
+		input = append(input, fmt.Sprintf("KEY%d=VALUE%d", i, i))
+	}
+	out := map[string]string{}
+	err := p.parse(strings.Join(input, "\n"), out, nil)
+	assert.NilError(t, err)
+	assert.Equal(t, size, len(out))
+	runtime.ReadMemStats(&endMemStats)
+	assert.Assert(t, endMemStats.Alloc-startMemStats.Alloc < uint64(size)*1000, /* assume 1K per line */
+		"memory usage should be linear with input size. Memory grew by: %d",
+		endMemStats.Alloc-startMemStats.Alloc)
+}