What steps will reproduce the problem?
1. Running a heavily utilized network service, like a crawler with 100 threads.
2. Call transport.CloseIdleConnections(), which nils the idleConn member.
3. At some point the transport's putIdleConn() function is called, resulting in a
runtime panic because the map is nil.

What is the expected output?

Not to panic - putIdleConn() should recreate the map like getIdleConn() does.

What do you see instead?

panic: runtime error: assignment to entry in nil map

goroutine 65 [running]:
net/http.(*Transport).putIdleConn(0xf8400bcdc0, 0xf8401c7f00, 0x0, 0xf8404edca6,
0xf84025cf18, ...)
    /home/michael/go/src/pkg/net/http/transport.go:263 +0x1c7
net/http._func_009(0xf84025cf18, 0xf84025ccf8, 0xf84025cb18, 0x44a2ce, 0xf840de81c0, ...)
    /home/michael/go/src/pkg/net/http/transport.go:575 +0x33
net/http.(*bodyEOFSignal).Close(0xf8405bb3c0, 0x0, 0x0, 0x0)
    /home/michael/go/src/pkg/net/http/transport.go:719 +0xb6
main.(*FilePoster).posterThread(0xf84008f780, 0xf8400ee780, 0xf80000003d, 0x0, 0x0, ...)
    /home/michael/gocode/src/concur/autoposter/httppost.go:94 +0x1b29
created by main.(*FilePoster).Start
    /home/michael/gocode/src/concur/autoposter/fileposter.go:64 +0x349

Which compiler are you using (5g, 6g, 8g, gccgo)?

6g

Which operating system are you using?

Ubuntu Linux

Which revision are you using?  (hg identify)

f4470a54e6db weekly/weekly.2012-03-04

Please provide any additional information below.

I inspected the latest version of /src/pkg/net/http/transport.go and noticed the issue
while investigating the panic. The panic does not occur if you don't call
CloseIdleConnections().

If you add:

if t.idleConn == nil {
  t.idleConn = make(map[string][]*persistConn)
}

at line 247 in transport.go I believe it will fix it.