internal/graph: Escape labels to support double quotes

[evanj]

Fixes syntax errors when trying to render pprof profiles that have
double quotes in tags. These can be created with Go's pprof labels
feature, for example with:

Fixes syntax errors when trying to render pprof profiles that have
double quotes in tags. These can be created with Go's pprof labels
feature, for example with:

pprof.Labels("key", "label "double quote"\nline two")

Trying to display a graph generated with this lable will fail:

Error: : syntax error in line 5 near 'quote'

The double quote (") was never escaped in the label strings. Add
a new escaping function that replaces newlines with centered lines
(\n) because the existing one replaces newline with left-justified
lines (\l).

[evanj]

This is a second attempt at #683. I started this as a new pull request because it takes a slightly different approach, so I was personally finding the commit history confusing instead of helpful.

[evanj]

I could pull the common parts of these two functions out into a single shared function, if that seems better.

[codecov-commenter]

Codecov Report

Merging #689 (34b8741) into main (c488b8f) will increase coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #689      +/-   ##
==========================================
+ Coverage   63.70%   63.78%   +0.07%     
==========================================
  Files          41       41              
  Lines        6489     6492       +3     
==========================================
+ Hits         4134     4141       +7     
+ Misses       1913     1908       -5     
- Partials      442      443       +1     

Impacted Files	Coverage Δ
...github.com/google/pprof/internal/graph/dotgraph.go	90.32% <0.00%> (+0.04%)	⬆️
...rc/github.com/google/pprof/internal/graph/graph.go	29.10% <0.00%> (+1.19%)	⬆️

---

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 c488b8f...34b8741. Read the comment docs.

[aalexand]

Replacing \n with \n seems a no-op.

[evanj]

The original escapeForDot function used the backticks version for \l, so I just modified it. The quote characters are different: "\n" is an interpreted string, so is a single byte newline. The backticks version is a raw string, so it the two byte sequence '\\' + 'n'.

[aalexand]

I might be missing something but quoting myself from #683:

I'd also expect any non-printable characters like newlines to be rendered as their C escaped version, so that the tag value is always one line when rendered. Same for function names. The escaping was initially added in #557 for graph legend labels that come from the profile comments field and for that we did support newlines to render newlines from comments as newlines since some comments intentionally use that kind of formatting. I think for function names and tag values it should be different and we shouldn't allow any formatting sequences in those strings.

So for function names and tag values I'd expect the newline to be escaped into a printable character, not rendered as either (left or center) actual newline character...

[evanj]

The newline handling is confusing, and there could be a better place to put it. However, I believe this version produces the desired output. Here is an example with my Go test output that currently fails to render with pprof, which shows the newlines in the key/value pairs being displayed as \n:

I tried to change the existing logic as little as possible. My logic was the following:

The joinLabels function combines multiple key/value pairs, and returns a string as it should be displayed to a human, without any dot/graphviz details. It replaces newline characters with a \n sequence, since we want a key/value pair to be on a single line. However, it then combines the multiple key/value pairs with newlines, since we want to display separate key/value pairs on separate lines. As a result, the new lines that remain should be "visible." The TestJoinLabels attempts to demonstrate this.

Next, escapeForDotCentered must convert this "human string" into a version that can be embedded into Graphviz's "escString". To do that, it must escape backslashes, escape quotes, and replace newline characters with \n. It turns out: I can remove the newline escaping from this function, since "centered text" in graphviz's default. However, this will make this function different from escapeForDot. Is that desirable?

I totally forgot about other non-printable characters though, so thanks! I will add that to joinLabels. My logic is if we were to us a different graph rendering tool, we would still want this escaping, so it should not be "dot specific".

[evanj]

I believe this PR should fix the bug that was just reported to the Go project: golang/go#53703 . The outstanding issue that I need to get back to is escaping non-printable characters, and adding a test for that case. I may not have time to get to that immediately. I think this PR doesn't make the handling of non-printable characters worse than it is today, but it would be nice to make this a "complete" fix. I will attempt to return to this change in the next couple of weeks.