Pad error text for browsers that are too smart.

404 page not found

Chrome would ignore this error page if this text weren't here.
Chrome would ignore this error page if this text weren't here.
Chrome would ignore this error page if this text weren't here.
Chrome would ignore this error page if this text weren't here.
Chrome would ignore this error page if this text weren't here.
Chrome would ignore this error page if this text weren't here.
Chrome would ignore this error page if this text weren't here.
Chrome would ignore this error page if this text weren't here.
Chrome would ignore this error page if this text weren't here.
Chrome would ignore this error page if this text weren't here.
Chrome would ignore this error page if this text weren't here.
Chrome would ignore this error page if this text weren't here.
Chrome would ignore this error page if this text weren't here.
Chrome would ignore this error page if this text weren't here.
Chrome would ignore this error page if this text weren't here.
Chrome would ignore this error page if this text weren't here.

R=presotto
APPROVED=p
DELTA=50  (50 added, 0 deleted, 0 changed)
OCL=30056
CL=30061

src/lib/http/server.go

@@ -21,6 +21,7 @@ import (
 	"os";
 	"path";
 	"strconv";
+	"strings";
 )
 
 // Errors introduced by the HTTP server.
@@ -53,6 +54,8 @@ type Conn struct {
 	chunking bool;	// using chunked transfer encoding for reply body
 	wroteHeader bool;	// reply header has been written
 	header map[string] string;	// reply header parameters
+	written int64;	// number of bytes written in body
+	status int;	// status code passed to WriteHeader
 }
 
 // Create new connection from rwc.
@@ -134,6 +137,8 @@ func (c *Conn) WriteHeader(code int) {
 		return
 	}
 	c.wroteHeader = true;
+	c.status = code;
+	c.written = 0;
 	if !c.Req.ProtoAtLeast(1, 0) {
 		return
 	}
@@ -168,6 +173,8 @@ func (c *Conn) Write(data []byte) (n int, err os.Error) {
 		return 0, nil
 	}
 
+	c.written += int64(len(data));	// ignoring errors, for errorKludge
+
 	// TODO(rsc): if chunking happened after the buffering,
 	// then there would be fewer chunk headers.
 	// On the other hand, it would make hijacking more difficult.
@@ -187,10 +194,53 @@ func (c *Conn) Write(data []byte) (n int, err os.Error) {
 	return n, err;
 }
 
+// If this is an error reply (4xx or 5xx)
+// and the handler wrote some data explaining the error,
+// some browsers (i.e., Chrome, Internet Explorer)
+// will show their own error instead unless the error is
+// long enough.  The minimum lengths used in those
+// browsers are in the 256-512 range.
+// Pad to 1024 bytes.
+func errorKludge(c *Conn, req *Request) {
+	const min = 1024;
+
+	// Is this an error?
+	if kind := c.status/100; kind != 4 && kind != 5 {
+		return;
+	}
+
+	// Did the handler supply any info?  Enough?
+	if c.written == 0 || c.written >= min {
+		return;
+	}
+
+	// Is it text?  ("Content-Type" is always in the map)
+	if s := c.header["Content-Type"]; len(s) < 5 || s[0:5] != "text/" {
+		return;
+	}
+
+	// Is it a broken browser?
+	var msg string;
+	switch agent := req.UserAgent; {
+	case strings.Index(agent, "MSIE") >= 0:
+		msg = "Internet Explorer";
+	case strings.Index(agent, "Chrome/") >= 0:
+		msg = "Chrome";
+	default:
+		return;
+	}
+	msg += " would ignore this error page if this text weren't here.\n";
+	io.WriteString(c, "\n");
+	for c.written < min {
+		io.WriteString(c, msg);
+	}
+}
+
 func (c *Conn) flush() {
 	if !c.wroteHeader {
 		c.WriteHeader(StatusOK);
 	}
+	errorKludge(c, c.Req);
 	if c.chunking {
 		io.WriteString(c.buf, "0\r\n");
 		// trailer key/value pairs, followed by blank line