No template function for JSON string escaping

[adeverteuil]

I tried using the printf "%q" function in go to encode double-quoted strings (like this: {{ printf "%q" .Annotations.description }}). This renders correct JSON strings in most cases, but it's not perfect.

I wasn't able to figure out how to use the jq template function. alertmanager-webhook-proxy crashes when I try to load the template.
I tried {{ jq .Labels.example }} and {{ .Labels.example | jq }}.

Reference documentation:

• text/template global functions:
  https://pkg.go.dev/text/template#hdr-Functions
• fmt.Sprintf syntax:
  https://pkg.go.dev/fmt#hdr-Printing
• JSON string specification:
  https://datatracker.ietf.org/doc/html/rfc8259#section-7
• Go string literals:
  https://go.dev/ref/spec#String_literals

Similarities between Go string literals and JSON strings:

• Double quotes delimiters.
• Two character escape sequences \\, \", \n, \t, and others.

Differences:

• Go literal strings have \nnn (octal), \xFF, \uFFFF and \UFFFFFFFF character sequences.
• JSON strings only have \uFFFF and a few 2-character sequences.

For example, in Go printf "%q" a null character will be represented as \x00, but JSON would require \u0000.

In JSON, all control characters U+0000 to U+001F must be escaped. Therefore, any control character that would not be represented as a 2-character escape sequence would produce bad JSON and cause an error.

Resolution:

I ended up re-writing in Python (because I am more familiar with that) so that I can use a JSON serialiser instead of a text template engine. This way, at least the JSON syntax is always correct.