binder: allow binding to a nil map

[georgmu]

Given the following code:

var params map[string]interface{} // <- this is nil
c.Bind(&params)

Currently, if the context does not define any path or query params, this might work when parsing a JSON body.
If there are path parameters involved, this will panic with:

echo: http: panic serving 127.0.0.1:41198: assignment to entry in nil map
goroutine 6 [running]:
net/http.(*conn).serve.func1()
	/usr/lib/golang/src/net/http/server.go:1868 +0x13d
panic({0x845340?, 0x8deaf0?})
	/usr/lib/golang/src/runtime/panic.go:920 +0x290
reflect.mapassign_faststr0(0x842580, 0x100c0000f8b18?, {0x8dc8c0?, 0x0?}, 0x842580?)
	/usr/lib/golang/src/runtime/map.go:1376 +0x25
reflect.mapassign_faststr(0x842580, 0x0, {0x8dc8c0, 0x1}, 0xc000017520)
	/usr/lib/golang/src/reflect/value.go:3837 +0x65
reflect.Value.SetMapIndex({0x842580, 0xc00006a088, 0x195}, {0x8327e0, 0xc000017510, 0x98}, {0x83d3a0, 0xc000017520, 0x94})
	/usr/lib/golang/src/reflect/value.go:2402 +0x2e5
github.com/labstack/echo/v4.(*DefaultBinder).bindData(0xad1780, {0x82ed80, 0xc00006a088}, 0xc0000f91b8, {0x89016d, 0x5})
	/home/georg/go/pkg/mod/github.com/labstack/echo/[email protected]/bind.go:152 +0x150d
github.com/labstack/echo/v4.(*DefaultBinder).BindPathParams(0xad1780, {0x8e42b8, 0xc0000aaa00}, {0x82ed80, 0xc00006a088})
	/home/georg/go/pkg/mod/github.com/labstack/echo/[email protected]/bind.go:40 +0x33d
github.com/labstack/echo/v4.(*DefaultBinder).Bind(0xad1780, {0x82ed80, 0xc00006a088}, {0x8e42b8, 0xc0000aaa00})
	/home/georg/go/pkg/mod/github.com/labstack/echo/[email protected]/bind.go:111 +0x6d
github.com/labstack/echo/v4.(*context).Bind(0xc0000aaa00, {0x82ed80, 0xc00006a088})
	/home/georg/go/pkg/mod/github.com/labstack/echo/[email protected]/context.go:439 +0x5d

With this patch applied, there are no panics anymore.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (60fc2fb) 93.02% compared to head (b821001) 93.02%.
Report is 6 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2574   +/-   ##
=======================================
  Coverage   93.02%   93.02%           
=======================================
  Files          39       39           
  Lines        4673     4676    +3     
=======================================
+ Hits         4347     4350    +3     
  Misses        236      236           
  Partials       90       90           

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[aldas]

I think this PR is overall OK but that check could be moved. If you have time please change that and if not, I can do it myself this weekend and merge it.

[algorithmcardboard]

I am running into this issue as well. Thank you @georgmu for the fix. @aldas any idea when this will make it to the release?

[aldas]

@algorithmcardboard , I really can not say at the moment, but you can use dest := map[string]int{} syntax to create your maps instead of var dest map[string]int

[aldas]

@georgmu thanks for the fix and @algorithmcardboard for pinging me. I forgot this PR again.