more comments about buffering and add new tests

modules/caddyhttp/reverseproxy/buffering_test.go

@@ -0,0 +1,84 @@
+package reverseproxy
+
+import (
+	"io"
+	"testing"
+)
+
+type zeroReader struct{}
+
+func (zeroReader) Read(p []byte) (int, error) {
+	for i := range p {
+		p[i] = 0
+	}
+	return len(p), nil
+}
+
+func TestBuffering(t *testing.T) {
+	var (
+		h  Handler
+		zr zeroReader
+	)
+	type args struct {
+		body  io.ReadCloser
+		limit int64
+	}
+	tests := []struct {
+		name        string
+		args        args
+		resultCheck func(io.ReadCloser, int64, args) bool
+	}{
+		{
+			name: "0 limit, body is returned as is",
+			args: args{
+				body:  io.NopCloser(&zr),
+				limit: 0,
+			},
+			resultCheck: func(res io.ReadCloser, read int64, args args) bool {
+				return res == args.body && read == args.limit && read == 0
+			},
+		},
+		{
+			name: "negative limit, body is read completely",
+			args: args{
+				body:  io.NopCloser(io.LimitReader(&zr, 100)),
+				limit: -1,
+			},
+			resultCheck: func(res io.ReadCloser, read int64, args args) bool {
+				brc, ok := res.(bodyReadCloser)
+				return ok && brc.body == nil && brc.buf.Len() == 100 && read == 100
+			},
+		},
+		{
+			name: "positive limit, body is read partially",
+			args: args{
+				body:  io.NopCloser(io.LimitReader(&zr, 100)),
+				limit: 50,
+			},
+			resultCheck: func(res io.ReadCloser, read int64, args args) bool {
+				brc, ok := res.(bodyReadCloser)
+				return ok && brc.body != nil && brc.buf.Len() == 50 && read == 50
+			},
+		},
+		{
+			name: "positive limit, body is read completely",
+			args: args{
+				body:  io.NopCloser(io.LimitReader(&zr, 100)),
+				limit: 101,
+			},
+			resultCheck: func(res io.ReadCloser, read int64, args args) bool {
+				brc, ok := res.(bodyReadCloser)
+				return ok && brc.body == nil && brc.buf.Len() == 100 && read == 100
+			},
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			res, read := h.bufferedBody(tt.args.body, tt.args.limit)
+			if !tt.resultCheck(res, read, tt.args) {
+				t.Error("Handler.bufferedBody() test failed")
+				return
+			}
+		})
+	}
+}

modules/caddyhttp/reverseproxy/reverseproxy.go

@@ -1221,6 +1221,10 @@ func (h Handler) provisionUpstream(upstream *Upstream) {
 // then returns a reader for the buffer along with how many bytes were buffered. Always close
 // the return value when done with it, just like if it was the original body! If limit is 0
 // (which it shouldn't be), this function returns its input; i.e. is a no-op, for safety.
+// Otherwise, it returns bodyReadCloser, the original body will be closed and body will be nil
+// if it's explicitly configured to buffer all or EOF is reached when reading.
+// TODO: the error during reading is discarded if the limit is negative, should the error be propagated
+// to upstream/downstream?
 func (h Handler) bufferedBody(originalBody io.ReadCloser, limit int64) (io.ReadCloser, int64) {
 	if limit == 0 {
 		return originalBody, 0