mime/multipart: fix ReadForm always return (nil,io.EOF)ms-fix-multipart-readform

Previously, the condition err == io.EOF in readForm will never true and
break; it always goes to the second condition, err != nil, which
cause the returned ReadForm always nil with err is io.EOF.

As the test, we use the example with body almost similar to
ExampleNewReader with header contains "Content-Disposition:form-data".

Change-Id: I7268f45bb26eafb7f1e6e471b86eec681dde99f7

4 files changed, 50 insertions, 6 deletions

diff --git a/src/mime/multipart/example_test.go b/src/mime/multipart/example_test.go
index fe154ac4f6..7a22ff789e 100644
--- a/src/mime/multipart/example_test.go
+++ b/src/mime/multipart/example_test.go
@@ -50,3 +50,43 @@ func ExampleNewReader() {
 	// Part "one": "A section"
 	// Part "two": "And another"
 }
+
+func ExampleReader_ReadForm() {
+	body := strings.NewReader(
+		"--foo\r\nContent-Disposition: form-data; name=\"field1\"\r\n\r\n" +
+			"Value of field.\r\n" +
+			"--foo\r\nContent-Disposition: form-data; name=\"file1\"; filename=\"file1.txt\"\r\n\r\n" +
+			"Value of file1.\r\n" +
+			"--foo--\r\n")
+
+	mr := multipart.NewReader(body, "foo")
+	form, err := mr.ReadForm(0)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	for name, val := range form.Value {
+		fmt.Printf("Form.Value %q: %q\n", name, val)
+	}
+	for name, listHeader := range form.File {
+		for _, hdr := range listHeader {
+			file, err := hdr.Open()
+			if err != nil {
+				log.Fatal(err)
+			}
+			content, err := io.ReadAll(file)
+			if err != nil {
+				log.Fatal(err)
+			}
+			fmt.Printf("Form.File %q:\n", name)
+			fmt.Printf("  Filename: %q\n", hdr.Filename)
+			fmt.Printf("  Content: %q\n", content)
+		}
+	}
+
+	// Output:
+	// Form.Value "field1": ["Value of field."]
+	// Form.File "file1":
+	//   Filename: "file1.txt"
+	//   Content: "Value of file1."
+}
diff --git a/src/mime/multipart/formdata.go b/src/mime/multipart/formdata.go
index f8258a961c..f2ca0118a6 100644
--- a/src/mime/multipart/formdata.go
+++ b/src/mime/multipart/formdata.go
@@ -108,7 +108,7 @@ func (r *Reader) readForm(maxMemory int64) (_ *Form, err error) {
 	var copyBuf []byte
 	for {
 		p, err := r.nextPart(false, maxMemoryBytes, maxHeaders)
-		if err == io.EOF {
+		if errors.Is(err, io.EOF) {
 			break
 		}
 		if err != nil {
diff --git a/src/mime/multipart/multipart.go b/src/mime/multipart/multipart.go
index da1f45810e..c570f1d7e2 100644
--- a/src/mime/multipart/multipart.go
+++ b/src/mime/multipart/multipart.go
@@ -30,6 +30,7 @@ package multipart
 import (
 	"bufio"
 	"bytes"
+	"errors"
 	"fmt"
 	"internal/godebug"
 	"io"
@@ -392,7 +393,7 @@ func (r *Reader) nextPart(rawPart bool, maxMIMEHeaderSize, maxMIMEHeaders int64)
 	for {
 		line, err := r.bufReader.ReadSlice('\n')
 
-		if err == io.EOF && r.isFinalBoundary(line) {
+		if errors.Is(err, io.EOF) && r.isFinalBoundary(line) {
 			// If the buffer ends in "--boundary--" without the
 			// trailing "\r\n", ReadSlice will return an error
 			// (since it's missing the '\n'), but this is a valid
diff --git a/src/net/http/request_test.go b/src/net/http/request_test.go
index 78b968f23c..c952b0ad26 100644
--- a/src/net/http/request_test.go
+++ b/src/net/http/request_test.go
@@ -251,14 +251,17 @@ func TestParseMultipartForm(t *testing.T) {
 		Body:   io.NopCloser(new(bytes.Buffer)),
 	}
 	err := req.ParseMultipartForm(25)
-	if err == nil {
-		t.Error("expected multipart EOF, got nil")
+	if err != nil {
+		t.Errorf("expecting nil, got %v", err)
 	}
 
+	// Reset to allow the call to multipartReader.
+	req.MultipartForm = nil
+
 	req.Header = Header{"Content-Type": {"text/plain"}}
 	err = req.ParseMultipartForm(25)
-	if err != ErrNotMultipart {
-		t.Error("expected ErrNotMultipart for text/plain")
+	if !errors.Is(err, ErrNotMultipart) {
+		t.Errorf("expected ErrNotMultipart for text/plain, got %v", err)
 	}
 }