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doc: DEVFAQ: describe latest not-too-good state of ghc-debug support

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Simon Michael 2024-05-01 16:21:24 -10:00
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doc/DEVFAQ.md
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@ -54,26 +54,34 @@ $ cabal install all:exes
### How do I build/run with ghc-debug support ?
You might need to stop background builders like HLS, to avoid a fight over the build flag
(in VS Code, run the command "Haskell: Stop Haskell LSP server").
Then build lib and executable(s) with the `ghcdebug` flag:
```cli
$ stack build --flag hledger-lib:ghcdebug --flag hledger:ghcdebug --flag hledger-ui:ghcdebug --flag hledger-web:ghcdebug
```
Use --flag to enable `ghcdebug` for hledger-lib and any of the executables you want to build, as above.
You might need to stop background builders like HLS while you do this, to avoid a fight over the build flag
(in VS Code, do Haskell: Stop Haskell LSP Server).
Your --version output should now say "supports ghc-debug":
When the build is right, --version should mention ghc-debug:
```
$ stack exec -- hledger --version
... with ghc debug support
```
When a hledger executable supports ghc-debug, you can enable the ghc-debug listener by running with a negative --debug level. Eg:
Now if you run at debug level -1, -2 or -3 the output should mention ghc-debug:
```cli
$ hledger CMD --debug=-1 # run normally, while also listening for ghc-debug commands
$ hledger CMD --debug=-1 # run normally, and listen for ghc-debug commands
$ hledger CMD --debug=-2 # pause for ghc-debug commands at program start
$ hledger CMD --debug=-3 # pause for ghc-debug commands at program end
Starting ghc-debug on socket: ...
```
You should see "Starting ghc-debug on socket: ..." output.
Then in another window, run the interactive `ghc-debug-brick` client, select the running hledger process,
and explore memory/profile information, resume/pause execution, etc.
Or you can write a custom ghc-debug-client script to extract specific information.
Then in another window, you can run [ghc-debug-brick](https://hackage.haskell.org/package/ghc-debug-brick) and it will show the running hledger process. Press enter to connect. This is not robust. Tips:
- you might need to clear out stale sockets: `rm -f ~/.local/share/ghc-debug/debuggee/sockets/*`
- you might need to kill stale hledger processes: `pkill -fl hledger`
- it might still fail with this error, reasons unclear:\
`rts_resume: called from a different OS thread than rts_pause`
- if it connects and the program is running, press `p` to pause it
At this point, you can explore memory/profile information, save snapshots, resume execution, etc.
Or, instead of ghc-debug-brick you can write a [ghc-debug-client](https://hackage.haskell.org/package/ghc-debug-client) script to extract specific information.