fix Context.Next() - recheck len of handlers every iteration

[vkd]

If gin sometimes reset Context.index and .handlers then needs to recheck len of handlers every iteration on func Context.Next():

gin/context.go

Lines 103 to 111 in 29a145c

	// Next should be used only inside middleware.
	// It executes the pending handlers in the chain inside the calling handler.
	// See example in GitHub.
	func (c *Context) Next() {
	c.index++
	for s := int8(len(c.handlers)); c.index < s; c.index++ {
	c.handlers[c.index](c)
	}
	}

---

Fix #1678, because gin change c.handlers when error on open file.

gin/routergroup.go

Lines 195 to 202 in 29a145c

	// Check if file exists and/or if we have permission to access it
	if _, err := fs.Open(file); err != nil {
	c.Writer.WriteHeader(http.StatusNotFound)
	c.handlers = group.engine.allNoRoute
	// Reset index
	c.index = -1
	return
	}

---

Fix #1744, because gin reset c.handlers slice on HandleContext() func.

gin/gin.go

Lines 354 to 360 in 29a145c

	// HandleContext re-enter a context that has been rewritten.
	// This can be done by setting c.Request.URL.Path to your new target.
	// Disclaimer: You can loop yourself to death with this, use wisely.
	func (engine *Engine) HandleContext(c *Context) {
	c.reset()
	engine.handleHTTPRequest(c)
	}

gin/context.go

Lines 67 to 75 in 29a145c

	func (c *Context) reset() {
	c.Writer = &c.writermem
	c.Params = c.Params[0:0]
	c.handlers = nil
	c.index = -1
	c.Keys = nil
	c.Errors = c.Errors[0:0]
	c.Accepted = nil
	}

[codecov]

Codecov Report

Merging #1745 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1745      +/-   ##
==========================================
+ Coverage   98.48%   98.48%   +<.01%     
==========================================
  Files          41       41              
  Lines        2041     2042       +1     
==========================================
+ Hits         2010     2011       +1     
  Misses         19       19              
  Partials       12       12

Impacted Files	Coverage Δ
context.go	98.3% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b056a34...676c7b2. Read the comment docs.

[thinkerou]

thanks @vkd can you add test case?

[thinkerou]

use while style?

[thinkerou]

LGTM, thanks!