Panic on bad path via API

[Stebalien]

I'm seeing the following error (produced by the daemon) when manually calling the API with a bad path (and asking for a protobuf output):

> curl -i "http://localhost:5001/api/v0/object/get?arg=&encoding=protobuf"

20:37:12.107 ERROR  cmds/http: expected type <-chan interface {}, got <nil> responseemitter.go:124
20:37:12.107 ERROR  cmds/http: a panic has occurred in the commands handler! handler.go:125
20:37:12.107 ERROR  cmds/http: expected type <-chan interface {}, got <nil> handler.go:126
20:37:12.108 ERROR  cmds/http: stack trace:
goroutine 929199 [running]:
runtime/debug.Stack(0xc42000d520, 0xc42e61f890, 0x1)
        /usr/lib/go/src/runtime/debug/stack.go:24 +0xa7
gx/ipfs/QmamUWYjFeYYzFDFPTvnmGkozJigsoDWUA4zoifTRFTnwK/go-ipfs-cmds/http.internalHandler.ServeHTTP.func1()
        /home/steb/projects/go/src/gx/ipfs/QmamUWYjFeYYzFDFPTvnmGkozJigsoDWUA4zoifTRFTnwK/go-ipfs-cmds/http/handler.go:127 +0x130
panic(0x12cd260, 0xc42e61f430)
        /usr/lib/go/src/runtime/panic.go:491 +0x283
gx/ipfs/QmamUWYjFeYYzFDFPTvnmGkozJigsoDWUA4zoifTRFTnwK/go-ipfs-cmds/http.(*responseEmitter).SetError(0xc4202a9880, 0x12cd260, 0xc420073960, 0x0)
        /home/steb/projects/go/src/gx/ipfs/QmamUWYjFeYYzFDFPTvnmGkozJigsoDWUA4zoifTRFTnwK/go-ipfs-cmds/http/responseemitter.go:147 +0x228
gx/ipfs/QmamUWYjFeYYzFDFPTvnmGkozJigsoDWUA4zoifTRFTnwK/go-ipfs-cmds.(*fakeResponse).SetError(0xc423edadc0, 0x1cb3280, 0xc420073960, 0x0)
        /home/steb/projects/go/src/gx/ipfs/QmamUWYjFeYYzFDFPTvnmGkozJigsoDWUA4zoifTRFTnwK/go-ipfs-cmds/legacy.go:191 +0xa6
github.com/ipfs/go-ipfs/core/commands/object.glob..func4(0x1cc95c0, 0xc42e61f0e0, 0x1cc8620, 0xc423edadc0)
        /home/steb/projects/go/src/github.com/ipfs/go-ipfs/core/commands/object/object.go:221 +0x517
gx/ipfs/QmamUWYjFeYYzFDFPTvnmGkozJigsoDWUA4zoifTRFTnwK/go-ipfs-cmds.NewCommand.func1(0x1cc9500, 0xc421992480, 0x6723494dcd68, 0xc4202a9880)
        /home/steb/projects/go/src/gx/ipfs/QmamUWYjFeYYzFDFPTvnmGkozJigsoDWUA4zoifTRFTnwK/go-ipfs-cmds/legacy.go:405 +0x18c
gx/ipfs/QmamUWYjFeYYzFDFPTvnmGkozJigsoDWUA4zoifTRFTnwK/go-ipfs-cmds.(*Command).Call(0x1c98660, 0x1cc9500, 0xc421992480, 0x6723494dcd68, 0xc4202a9880, 0x0, 0x0)
        /home/steb/projects/go/src/gx/ipfs/QmamUWYjFeYYzFDFPTvnmGkozJigsoDWUA4zoifTRFTnwK/go-ipfs-cmds/command.go:118 +0x1cb
gx/ipfs/QmamUWYjFeYYzFDFPTvnmGkozJigsoDWUA4zoifTRFTnwK/go-ipfs-cmds/http.internalHandler.ServeHTTP(0x0, 0xc42001804a, 0xd, 0xc42027ea80, 0xc4201cc240, 0x1573708, 0xc4201be700, 0xc420382fa0, 0x1c98660, 0xc42027e5a0, ...)
        /home/steb/projects/go/src/gx/ipfs/QmamUWYjFeYYzFDFPTvnmGkozJigsoDWUA4zoifTRFTnwK/go-ipfs-cmds/http/handler.go:193 +0x9c3
gx/ipfs/QmamUWYjFeYYzFDFPTvnmGkozJigsoDWUA4zoifTRFTnwK/go-ipfs-cmds/http.(*internalHandler).ServeHTTP(0xc4204f3b80, 0x1cbdb00, 0xc4204f8358, 0xc420356100)
        <autogenerated>:1 +0x72
gx/ipfs/QmPG2kW5t27LuHgHnvhUwbHCNHAt2eUcb4gPHqofrESUdB/cors.(*Cors).Handler.func1(0x1cbdb00, 0xc4204f8358, 0xc420356100)
        /home/steb/projects/go/src/gx/ipfs/QmPG2kW5t27LuHgHnvhUwbHCNHAt2eUcb4gPHqofrESUdB/cors/cors.go:188 +0x108
net/http.HandlerFunc.ServeHTTP(0xc4202c4c00, 0x1cbdb00, 0xc4204f8358, 0xc420356100)
        /usr/lib/go/src/net/http/server.go:1918 +0x44
gx/ipfs/QmamUWYjFeYYzFDFPTvnmGkozJigsoDWUA4zoifTRFTnwK/go-ipfs-cmds/http.Handler.ServeHTTP(0x0, 0xc42001804a, 0xd, 0xc42027ea80, 0xc4201cc240, 0x1573708, 0x0, 0xc420382fa0, 0x1c98660, 0xc42027e5a0, ...)
        /home/steb/projects/go/src/gx/ipfs/QmamUWYjFeYYzFDFPTvnmGkozJigsoDWUA4zoifTRFTnwK/go-ipfs-cmds/http/handler.go:117 +0x5e
gx/ipfs/QmamUWYjFeYYzFDFPTvnmGkozJigsoDWUA4zoifTRFTnwK/go-ipfs-cmds/http.(*Handler).ServeHTTP(0xc420287ec0, 0x1cbdb00, 0xc4204f8358, 0xc420356100)
        <autogenerated>:1 +0x78
net/http.(*ServeMux).ServeHTTP(0xc420311800, 0x1cbdb00, 0xc4204f8358, 0xc420356100)
        /usr/lib/go/src/net/http/server.go:2254 +0x130
net/http.(Handler).ServeHTTP-fm(0x1cbdb00, 0xc4204f8358, 0xc420356100)
        /usr/lib/go/src/net/http/h2_bundle.go:5462 +0x4d
gx/ipfs/QmX3QZ5jHEPidwUrymXV1iSCSUhdGxj15sm2gP4jKMef7B/client_golang/prometheus.InstrumentHandlerFuncWithOpts.func1(0x1cbf080, 0xc42dc761c0, 0xc420356100)
        /home/steb/projects/go/src/gx/ipfs/QmX3QZ5jHEPidwUrymXV1iSCSUhdGxj15sm2gP4jKMef7B/client_golang/prometheus/http.go:287 +0x26f
net/http.HandlerFunc.ServeHTTP(0xc4204f32c0, 0x1cbf080, 0xc42dc761c0, 0xc420356100)
        /usr/lib/go/src/net/http/server.go:1918 +0x44
net/http.(*ServeMux).ServeHTTP(0xc4203117d0, 0x1cbf080, 0xc42dc761c0, 0xc420356100)
        /usr/lib/go/src/net/http/server.go:2254 +0x130
net/http.serverHandler.ServeHTTP(0xc4201b1520, 0x1cbf080, 0xc42dc761c0, 0xc420356100)
        /usr/lib/go/src/net/http/server.go:2619 +0xb4
net/http.(*conn).serve(0xc4285f6320, 0x1cbff00, 0xc4213c7740)
        /usr/lib/go/src/net/http/server.go:1801 +0x71d
created by net/http.(*Server).Serve
        /usr/lib/go/src/net/http/server.go:2720 +0x288
 handler.go:127

related to #2468

This is not fixed by ipfs/go-ipfs-cmds#34

[keks]

The problem here is that an error occurs and we can't encode that error using protobuf. When the error occurs we call SetError (at core/commands/object/object.go:221), which calls Emit, which tries to encode the error and fails with an uninformative error. SetError panics if Emit fails, which is what we see here.
I have some local changes that make the error more informative, but I'm not sure how we can keep complexity in bounds and not panic in the handler.

Digging through this made me wonder what happened before go-ipfs-cmds. Here is the output for 0.4.13:

15:47:55.443 ERROR commands/h: a panic has occurred in the commands handler! handler.go:124
15:47:55.444 ERROR commands/h: interface conversion: interface {} is nil, not *objectcmd.Node handler.go:125
goroutine 130 [running]:
runtime/debug.Stack(0x3e, 0xc4202854a0, 0xd23d3a)
	/home/keks/local/go/src/runtime/debug/stack.go:24 +0xa7
runtime/debug.PrintStack()
	/home/keks/local/go/src/runtime/debug/stack.go:16 +0x22
github.com/ipfs/go-ipfs/commands/http.internalHandler.ServeHTTP.func1()
	/home/keks/gopaths/ipfs/src/github.com/ipfs/go-ipfs/commands/http/handler.go:127 +0x129
panic(0x12b4c20, 0xc420dea280)
	/home/keks/local/go/src/runtime/panic.go:491 +0x283
github.com/ipfs/go-ipfs/core/commands/object.glob..func5(0x1c749c0, 0xc4205001c0, 0xc42001c0a5, 0x8, 0xc420198be8, 0xc42001c0a5)
	/home/keks/gopaths/ipfs/src/github.com/ipfs/go-ipfs/core/commands/object/object.go:238 +0x1cc
github.com/ipfs/go-ipfs/commands.(*response).Marshal(0xc4205001c0, 0x0, 0x0, 0x0, 0x0)
	/home/keks/gopaths/ipfs/src/github.com/ipfs/go-ipfs/commands/response.go:194 +0x16a
github.com/ipfs/go-ipfs/commands.(*response).Reader(0xc4205001c0, 0xc420dee0a0, 0x13d83e9, 0x14, 0x0)
	/home/keks/gopaths/ipfs/src/github.com/ipfs/go-ipfs/commands/response.go:214 +0xd5
github.com/ipfs/go-ipfs/commands/http.sendResponse(0x1c6a140, 0xc420de2008, 0xc420df4000, 0x1c749c0, 0xc4205001c0, 0x1c75740, 0xc420dfe180)
	/home/keks/gopaths/ipfs/src/github.com/ipfs/go-ipfs/commands/http/handler.go:235 +0x125
github.com/ipfs/go-ipfs/commands/http.internalHandler.ServeHTTP(0x0, 0xc420092b10, 0x10, 0xc420202ab0, 0xc420266000, 0x1534b68, 0xc42014a1c0, 0xc4204fcc40, 0x1c41160, 0xc4202029c0, ...)
	/home/keks/gopaths/ipfs/src/github.com/ipfs/go-ipfs/commands/http/handler.go:193 +0x96c
github.com/ipfs/go-ipfs/commands/http.(*internalHandler).ServeHTTP(0xc420dc2190, 0x1c6a140, 0xc420de2008, 0xc420df4000)
	<autogenerated>:1 +0x72
gx/ipfs/QmPG2kW5t27LuHgHnvhUwbHCNHAt2eUcb4gPHqofrESUdB/cors.(*Cors).Handler.func1(0x1c6a140, 0xc420de2008, 0xc420df4000)
	/home/keks/gopaths/ipfs/src/gx/ipfs/QmPG2kW5t27LuHgHnvhUwbHCNHAt2eUcb4gPHqofrESUdB/cors/cors.go:188 +0x108
net/http.HandlerFunc.ServeHTTP(0xc420206460, 0x1c6a140, 0xc420de2008, 0xc420df4000)
	/home/keks/local/go/src/net/http/server.go:1918 +0x44
github.com/ipfs/go-ipfs/commands/http.Handler.ServeHTTP(0x0, 0xc420092b10, 0x10, 0xc420202ab0, 0xc420266000, 0x1534b68, 0x0, 0xc4204fcc40, 0x1c41160, 0xc4202029c0, ...)
	/home/keks/gopaths/ipfs/src/github.com/ipfs/go-ipfs/commands/http/handler.go:116 +0x5e
github.com/ipfs/go-ipfs/commands/http.(*Handler).ServeHTTP(0xc4201605a0, 0x1c6a140, 0xc420de2008, 0xc420df4000)
	<autogenerated>:1 +0x78
net/http.(*ServeMux).ServeHTTP(0xc420202030, 0x1c6a140, 0xc420de2008, 0xc420df4000)
	/home/keks/local/go/src/net/http/server.go:2254 +0x130
net/http.(Handler).ServeHTTP-fm(0x1c6a140, 0xc420de2008, 0xc420df4000)
	/home/keks/local/go/src/net/http/h2_bundle.go:5462 +0x4d
gx/ipfs/QmX3QZ5jHEPidwUrymXV1iSCSUhdGxj15sm2gP4jKMef7B/client_golang/prometheus.InstrumentHandlerFuncWithOpts.func1(0x1c6b700, 0xc4205260e0, 0xc420df4000)
	/home/keks/gopaths/ipfs/src/gx/ipfs/QmX3QZ5jHEPidwUrymXV1iSCSUhdGxj15sm2gP4jKMef7B/client_golang/prometheus/http.go:287 +0x26f
net/http.HandlerFunc.ServeHTTP(0xc420dc2140, 0x1c6b700, 0xc4205260e0, 0xc420df4000)
	/home/keks/local/go/src/net/http/server.go:1918 +0x44
net/http.(*ServeMux).ServeHTTP(0xc420202000, 0x1c6b700, 0xc4205260e0, 0xc420df4000)
	/home/keks/local/go/src/net/http/server.go:2254 +0x130
net/http.serverHandler.ServeHTTP(0xc420dda000, 0x1c6b700, 0xc4205260e0, 0xc420df4000)
	/home/keks/local/go/src/net/http/server.go:2619 +0xb4
net/http.(*conn).serve(0xc4205020a0, 0x1c6c580, 0xc420dea040)
	/home/keks/local/go/src/net/http/server.go:1801 +0x71d
created by net/http.(*Server).Serve
	/home/keks/local/go/src/net/http/server.go:2720 +0x288

[keks]

Branch with more informative error: https://github.com/ipfs/go-ipfs-cmds/tree/fix/cant-encode-error

[Stebalien]

Oh. THIS bug. #2517