[release-branch.go1.10] mime/multipart: restore 1.9 handling of missing/empty form-data file name

Revert the code change of CL 70931, but keep the test, appropriately
modified for the code changes. Also add a new test. This restores the
1.9 handling of form-data entries with missing or empty file names.

Changing the handling of this simply confused existing programs for no
useful benefit. Go back to the old behavior.

Updates #19183
Fixes #24041

Change-Id: Ie7a0309a061218ceda3bbc2a7da85e6fb3dd016d
Reviewed-on: https://go-review.googlesource.com/121075
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

2 files changed, 26 insertions, 6 deletions

diff --git a/src/mime/multipart/formdata.go b/src/mime/multipart/formdata.go
index 2a4ebdd4a0..832d0ad693 100644
--- a/src/mime/multipart/formdata.go
+++ b/src/mime/multipart/formdata.go
@@ -58,8 +58,7 @@ func (r *Reader) readForm(maxMemory int64) (_ *Form, err error) {
 
 		var b bytes.Buffer
 
-		_, hasContentTypeHeader := p.Header["Content-Type"]
-		if !hasContentTypeHeader && filename == "" {
+		if filename == "" {
 			// value, store as string in memory
 			n, err := io.CopyN(&b, p, maxValueBytes+1)
 			if err != nil && err != io.EOF {
diff --git a/src/mime/multipart/formdata_test.go b/src/mime/multipart/formdata_test.go
index 69333d3d0d..2d6a830cb6 100644
--- a/src/mime/multipart/formdata_test.go
+++ b/src/mime/multipart/formdata_test.go
@@ -47,12 +47,24 @@ func TestReadFormWithNamelessFile(t *testing.T) {
 	}
 	defer f.RemoveAll()
 
-	fd := testFile(t, f.File["hiddenfile"][0], "", filebContents)
-	if _, ok := fd.(sectionReadCloser); !ok {
-		t.Errorf("file has unexpected underlying type %T", fd)
+	if g, e := f.Value["hiddenfile"][0], filebContents; g != e {
+		t.Errorf("hiddenfile value = %q, want %q", g, e)
 	}
-	fd.Close()
+}
 
+func TestReadFormWithTextContentType(t *testing.T) {
+	// From https://github.com/golang/go/issues/24041
+	b := strings.NewReader(strings.Replace(messageWithTextContentType, "\n", "\r\n", -1))
+	r := NewReader(b, boundary)
+	f, err := r.ReadForm(25)
+	if err != nil {
+		t.Fatal("ReadForm:", err)
+	}
+	defer f.RemoveAll()
+
+	if g, e := f.Value["texta"][0], textaValue; g != e {
+		t.Errorf("texta value = %q, want %q", g, e)
+	}
 }
 
 func testFile(t *testing.T, fh *FileHeader, efn, econtent string) File {
@@ -94,6 +106,15 @@ Content-Type: text/plain
 --MyBoundary--
 `
 
+const messageWithTextContentType = `
+--MyBoundary
+Content-Disposition: form-data; name="texta"
+Content-Type: text/plain
+
+` + textaValue + `
+--MyBoundary
+`
+
 const message = `
 --MyBoundary
 Content-Disposition: form-data; name="filea"; filename="filea.txt"