From 8309ddd486b078f6e78011a357f6208a5f20a4e8 Mon Sep 17 00:00:00 2001
From: Klaus Post <klauspost@gmail.com>
Date: Thu, 2 Dec 2021 11:22:32 -0800
Subject: [PATCH] Fix panic (not fatal) on connection drops (#13811)

Fix more regressions from #13597 with double closed channels.

```
panic: "POST /minio/storage/data/distxl-plain/s1/d2/v42/createfile?disk-id=c789f7e1-2b52-442a-b518-aa2dac03f3a1&file-path=f6161668-b939-4543-9873-91b9da4cdff6%2F5eafa986-a3bf-4b1c-8bc0-03a37de390a3%2Fpart.1&length=2621760&volume=.minio.sys%2Ftmp": send on closed channel
goroutine 1977 [running]:
runtime/debug.Stack()
        c:/go/src/runtime/debug/stack.go:24 +0x65
github.com/minio/minio/cmd.setCriticalErrorHandler.func1.1()
        d:/minio/minio/cmd/generic-handlers.go:468 +0x8e
panic({0x2928860, 0x4fb17e0})
        c:/go/src/runtime/panic.go:1038 +0x215
github.com/minio/minio/cmd.keepHTTPReqResponseAlive.func2({0x4fe4ea0, 0xc02737d8a0})
        d:/minio/minio/cmd/storage-rest-server.go:818 +0x48
github.com/minio/minio/cmd.(*storageRESTServer).CreateFileHandler(0xc0015a8510, {0x50073e0, 0xc0273ec460}, 0xc029b9a400)
        d:/minio/minio/cmd/storage-rest-server.go:334 +0x1d2
net/http.HandlerFunc.ServeHTTP(...)
        c:/go/src/net/http/server.go:2046
github.com/minio/minio/cmd.httpTraceHdrs.func1({0x50073e0, 0xc0273ec460}, 0x0)
        d:/minio/minio/cmd/handler-utils.go:372 +0x53
net/http.HandlerFunc.ServeHTTP(0x5007380, {0x50073e0, 0xc0273ec460}, 0x10)
        c:/go/src/net/http/server.go:2046 +0x2f
github.com/minio/minio/cmd.addCustomHeaders.func1({0x5007380, 0xc0273dcf00}, 0xc0273f7340)
```

Reverts but adds write checks.
---
 cmd/storage-rest-server.go | 73 ++++++++++++++++++++------------------
 1 file changed, 38 insertions(+), 35 deletions(-)

diff --git a/cmd/storage-rest-server.go b/cmd/storage-rest-server.go
index f79890716..56b3c82f6 100644
--- a/cmd/storage-rest-server.go
+++ b/cmd/storage-rest-server.go
@@ -760,51 +760,48 @@ func keepHTTPReqResponseAlive(w http.ResponseWriter, r *http.Request) (resp func
 	doneCh := make(chan error)
 	ctx := r.Context()
 	go func() {
-		defer close(doneCh)
+		var canWrite = true
+		write := func(b []byte) {
+			if canWrite {
+				n, err := w.Write(b)
+				if err != nil || n != len(b) {
+					canWrite = false
+				}
+			}
+		}
 		// Wait for body to be read.
 		select {
 		case <-ctx.Done():
-			return
 		case <-bodyDoneCh:
-		case err, ok := <-doneCh:
-			if !ok {
-				return
-			}
+		case err := <-doneCh:
 			if err != nil {
-				_, werr := w.Write([]byte{1})
-				if werr != nil {
-					return
-				}
-				w.Write([]byte(err.Error()))
+				write([]byte{1})
+				write([]byte(err.Error()))
 			} else {
-				w.Write([]byte{0})
+				write([]byte{0})
 			}
+			close(doneCh)
 			return
 		}
+		defer close(doneCh)
 		// Initiate ticker after body has been read.
 		ticker := time.NewTicker(time.Second * 10)
-		defer ticker.Stop()
 		for {
 			select {
 			case <-ticker.C:
 				// Response not ready, write a filler byte.
-				if _, err := w.Write([]byte{32}); err != nil {
-					return
-				}
-				w.(http.Flusher).Flush()
-			case err, ok := <-doneCh:
-				if !ok {
-					return
+				write([]byte{32})
+				if canWrite {
+					w.(http.Flusher).Flush()
 				}
+			case err := <-doneCh:
 				if err != nil {
-					_, werr := w.Write([]byte{1})
-					if werr != nil {
-						return
-					}
-					w.Write([]byte(err.Error()))
+					write([]byte{1})
+					write([]byte(err.Error()))
 				} else {
-					w.Write([]byte{0})
+					write([]byte{0})
 				}
+				ticker.Stop()
 				return
 			}
 		}
@@ -837,6 +834,15 @@ func keepHTTPReqResponseAlive(w http.ResponseWriter, r *http.Request) (resp func
 func keepHTTPResponseAlive(w http.ResponseWriter) func(error) {
 	doneCh := make(chan error)
 	go func() {
+		var canWrite = true
+		write := func(b []byte) {
+			if canWrite {
+				n, err := w.Write(b)
+				if err != nil || n != len(b) {
+					canWrite = false
+				}
+			}
+		}
 		defer close(doneCh)
 		ticker := time.NewTicker(time.Second * 10)
 		defer ticker.Stop()
@@ -844,19 +850,16 @@ func keepHTTPResponseAlive(w http.ResponseWriter) func(error) {
 			select {
 			case <-ticker.C:
 				// Response not ready, write a filler byte.
-				if _, err := w.Write([]byte{32}); err != nil {
-					return
+				write([]byte{32})
+				if canWrite {
+					w.(http.Flusher).Flush()
 				}
-				w.(http.Flusher).Flush()
 			case err := <-doneCh:
 				if err != nil {
-					_, werr := w.Write([]byte{1})
-					if werr != nil {
-						return
-					}
-					w.Write([]byte(err.Error()))
+					write([]byte{1})
+					write([]byte(err.Error()))
 				} else {
-					w.Write([]byte{0})
+					write([]byte{0})
 				}
 				return
 			}