-MJ Compilation Database with a single file produces invalid json

[FaultyPine]

Running clang with -MJ on my project, which uses a unity build file, produces the following (truncated for viewing ease) compile_commands.json.
{ "directory": "....", "file": "...", "output": "...", "arguments": [........] },
Note the trailing comma at the very end there, which breaks most json parsers, like nlohmann/json.
Though in general, the schema for this json is that there should be an array of these command objects, so really this should probably look like
[ { "directory": "....", ......, "arguments": [...] } ]
https://clang.llvm.org/docs/JSONCompilationDatabase.html
"A compilation database is a JSON file, which consist of an array of “command objects”"

[keinflue]

This is the intended behavior from what I can tell. The option doesn't produce a full compilation database, but only fragments/records that a build system can then concatenate into a JSON array which will form the full compilation database.

See commit adding the option cbc8725.

Also see https://clang.llvm.org/docs/ClangCommandLineReference.html:

-MJ

Write a compilation database entry per input
[emphasis mine]

As well as https://clang.llvm.org/docs/JSONCompilationDatabase.html:

Clang has the ability to generate compilation database fragments via -MJ argument <clang -MJ<arg>>. You can concatenate those fragments together between [ and ] to create a compilation database.