Report full stack traces when detecting leaked clients.

This patch adjusts the logic for detecting client leaks to include full
stack traces for where they were created, rather than just the source
location.

Author: zenhack

capability.go

@@ -7,6 +7,7 @@ import (
 	"strconv"
 	"sync"
 
+	"capnproto.org/go/capnp/v3/exp/bufferpool"
 	"capnproto.org/go/capnp/v3/flowcontrol"
 	"capnproto.org/go/capnp/v3/internal/syncutil"
 )
@@ -114,9 +115,10 @@ type ClientKind = struct {
 }
 
 type client struct {
-	creatorFunc int
-	creatorFile string
-	creatorLine int
+	creatorFunc  int
+	creatorFile  string
+	creatorStack string
+	creatorLine  int
 
 	mu       sync.Mutex // protects the struct
 	limiter  flowcontrol.FlowLimiter
@@ -147,6 +149,19 @@ type clientHook struct {
 	resolvedHook *clientHook // valid only if resolved is closed
 }
 
+func (c Client) setupLeakReporting(creatorFunc int) {
+	if clientLeakFunc == nil {
+		return
+	}
+	c.creatorFunc = creatorFunc
+	_, c.creatorFile, c.creatorLine, _ = runtime.Caller(2)
+	buf := bufferpool.Default.Get(1e6)
+	n := runtime.Stack(buf, false)
+	c.creatorStack = string(buf[:n])
+	bufferpool.Default.Put(buf)
+	c.setFinalizer()
+}
+
 // NewClient creates the first reference to a capability.
 // If hook is nil, then NewClient returns nil.
 //
@@ -165,11 +180,7 @@ func NewClient(hook ClientHook) Client {
 	}
 	h.resolvedHook = h
 	c := Client{client: &client{h: h}}
-	if clientLeakFunc != nil {
-		c.creatorFunc = 1
-		_, c.creatorFile, c.creatorLine, _ = runtime.Caller(1)
-		c.setFinalizer()
-	}
+	c.setupLeakReporting(1)
 	return c
 }
 
@@ -192,11 +203,7 @@ func NewPromisedClient(hook ClientHook) (Client, *ClientPromise) {
 		metadata:   *NewMetadata(),
 	}
 	c := Client{client: &client{h: h}}
-	if clientLeakFunc != nil {
-		c.creatorFunc = 2
-		_, c.creatorFile, c.creatorLine, _ = runtime.Caller(1)
-		c.setFinalizer()
-	}
+	c.setupLeakReporting(2)
 	return c, &ClientPromise{h: h}
 }
 
@@ -460,11 +467,7 @@ func (c Client) AddRef() Client {
 	c.h.refs++
 	c.h.mu.Unlock()
 	d := Client{client: &client{h: c.h}}
-	if clientLeakFunc != nil {
-		d.creatorFunc = 3
-		_, d.creatorFile, d.creatorLine, _ = runtime.Caller(1)
-		d.setFinalizer()
-	}
+	d.setupLeakReporting(3)
 	return d
 }
 
@@ -653,6 +656,9 @@ func finalizeClient(c *client) {
 	} else {
 		msg = fmt.Sprintf("leaked client created by %s on %s:%d", fname, c.creatorFile, c.creatorLine)
 	}
+	if c.creatorStack != "" {
+		msg += "\nCreation stack trace:\n" + c.creatorStack + "\n"
+	}
 
 	// finalizeClient will only be called if clientLeakFunc != nil.
 	go clientLeakFunc(msg)