Diff is empty when lines are too long

[taiyangc]

On assert.Equal failures, if the diff has long lines (due to complicated structs), instead of showing something like

require.go:157:
                	Error Trace:	common_test.go:72
                	            				transactions_test.go:844
                	Error:      	Not equal:
					expected: ...
					actual: ...

					Diff:
					--- Expected
					+++ Actual
					<super long line...>
                	Test:       	TestTransactionsQueryFingerprint
                	Messages:   	Response does not match expected return

Currently the diff is completely cut off

require.go:157:
                	Error Trace:	common_test.go:72
                	            				transactions_test.go:844
                	Error:      	Not equal:
                	Test:       	TestTransactionsQueryFingerprint
                	Messages:   	Response does not match expected return

This makes testing hard to check which lines differed.

Upon inspecting, indentation formatting of diff lines is using bufio.Scanner which has a default MaxScanTokenSize (64K) and would not parse long lines: https://github.com/stretchr/testify/blob/master/assert/assertions.go#L201

Can be fixed by either changing from bufio.Scanner to bufio.Reader or truncating long lines up to the max token size (or even provide a config to change max). Thoughts?

[abourget]

This is also blocking me.. any direction? I'll happily PR if the direction is given.

[xordspar0]

I added a failing test that demonstrates the issue. Someone can use this when working on a fix.

[mvdkleijn]

We don't really care for state and config very much and would like to avoid that in general so I'd not opt for making the max configurable.

For now it makes the most sense to me to display the whole line.

Another slightly more complex solution which might be preferable, would be to show a snippet of the line, indicating the difference and the column / character where the difference occurrs. That would prevent a huge line potentially messing up the output, making it unreadable.

[boyan-soubachov]

The change from bufio.Scanner to bufio.Reader seems like a good one that wouldn't break any compatibility. There was also a change added (truncatingFormat()) which truncates when the output is too long.

I'm happy to go with either approach. The latter probably makes more sense due to flooding the terminal with a massive text/diff dump makes for bad experience.

[mvdkleijn]

I missed the addition of truncatingFormat(). It's agreed then. We'll go for this solution:

show a snippet of the line, indicating the difference and the column / character where the difference occurs.

If anyone is willing to do a PR, that would help enormously. Volunteers?

[xordspar0]

I can't reproduce this anymore with the following test:

func TestTrucation(t *testing.T) {
	message := "a"
	for i:= 0; i < 17; i++ {
		message = message + message
	}


	assert.Equal(t, string(message), "b")
}

The test I provided in the linked PR still fails, but I think it's not valid to test indentMessageLines because normally the diff is already truncated by the time it's passed into indentMessageLines.

[malikov73]

Still reproducible on current master: truncatingFormat guards the expected:/actual: representations, but diff() output is not truncated, so e.g. comparing two long single-line strings (or structs with a long string field — spew prints strings on one line) still replaces the whole failure message with cannot display message: bufio.Scanner: token too long.

I've opened #1925 with a regression test demonstrating this and a minimal fix (no API changes, no scanner line-length limit). It doesn't preclude adding snippet-style truncation of long diff lines later, as discussed above.