Show breakpoint locations of breakpoints which were ignored during :force (#2950)

GHCi is split up into 2 major parts: The user-interface (UI)
and the byte-code interpreter. With `-fexternal-interpreter`
they even run in different processes. Communication between
the UI and the Interpreter (called `iserv`) is done using
messages over a pipe. This is called `Remote GHCI` and
explained in the Note [Remote GHCi] in `compiler/ghci/GHCi.hs`.

To process a `:force` command the UI sends a `Seq` message
to the `iserv` process. Then `iserv` does the effective
evaluation of the value. When during this process a breakpoint
is hit, the `iserv` process has no additional information to
enhance the `Ignoring breakpoint` output with the breakpoint
location.

To be able to print additional breakpoint information,
there are 2 possible implementation choices:
1. Store the needed information in the `iserv` process.
2. Print the `Ignoring breakpoint` from the UI process.

For option 1 we need to store the breakpoint info redundantely
in 2 places and this is bad. Therfore option 2 was implemented
in this MR:
- The user enters a `force` command
- The UI sends  a `Seq` message to the `iserv` process.
- If processing of the `Seq` message hits a breakpoint,
  the `iserv` process returns control to the UI process.
- The UI looks up the source location of the breakpoint,
  and prints the enhanced `Ignoring breakpoint` output.
- The UI sends a `ResumeSeq` message to the `iserv` process,
  to continue forcing.

6 files changed, 29 insertions, 1 deletions

diff --git a/testsuite/tests/ghci.debugger/scripts/T2950.script b/testsuite/tests/ghci.debugger/scripts/T2950.script
new file mode 100644
index 0000000000..bf10ed0d14
--- /dev/null
+++ b/testsuite/tests/ghci.debugger/scripts/T2950.script
@@ -0,0 +1,6 @@
+:l T2950M.hs
+:br 4 19
+:br 4 26
+:br T2950S 3
+main
+:force _result
diff --git a/testsuite/tests/ghci.debugger/scripts/T2950.stdout b/testsuite/tests/ghci.debugger/scripts/T2950.stdout
new file mode 100644
index 0000000000..d2401849e5
--- /dev/null
+++ b/testsuite/tests/ghci.debugger/scripts/T2950.stdout
@@ -0,0 +1,9 @@
+Breakpoint 0 activated at T2950M.hs:4:19-35
+Breakpoint 1 activated at T2950M.hs:4:26-35
+Breakpoint 2 activated at T2950S.hs:3:11-12
+Stopped in Main.main, T2950M.hs:4:19-35
+_result :: String = _
+*** Ignoring breakpoint [T2950M.hs:4:19-35]
+*** Ignoring breakpoint [T2950M.hs:4:26-35]
+*** Ignoring breakpoint [T2950S.hs:3:11-12]
+_result = "[2,6]"
diff --git a/testsuite/tests/ghci.debugger/scripts/T2950M.hs b/testsuite/tests/ghci.debugger/scripts/T2950M.hs
new file mode 100644
index 0000000000..fa49cda119
--- /dev/null
+++ b/testsuite/tests/ghci.debugger/scripts/T2950M.hs
@@ -0,0 +1,4 @@
+import T2950S
+
+main :: IO ()
+main = putStrLn $ show $ sort [6,2]
diff --git a/testsuite/tests/ghci.debugger/scripts/T2950S.hs b/testsuite/tests/ghci.debugger/scripts/T2950S.hs
new file mode 100644
index 0000000000..0685381f9f
--- /dev/null
+++ b/testsuite/tests/ghci.debugger/scripts/T2950S.hs
@@ -0,0 +1,8 @@
+module T2950S where
+sort :: Ord a => [a] -> [a]
+sort [] = []
+sort (x:xs) = insert x (sort xs)
+  where
+    insert x [] = [x]
+    insert x (y:ys) | x < y     = x:y:ys
+                    | otherwise = y:(insert x ys)
diff --git a/testsuite/tests/ghci.debugger/scripts/all.T b/testsuite/tests/ghci.debugger/scripts/all.T
index d38b3681ad..24939a942e 100644
--- a/testsuite/tests/ghci.debugger/scripts/all.T
+++ b/testsuite/tests/ghci.debugger/scripts/all.T
@@ -107,6 +107,7 @@ test('hist002', [extra_files(['../Test3.hs']), extra_run_opts('+RTS -I0')],
 test('T1620', extra_files(['T1620/', 'T1620/T1620.hs']),
               ghci_script, ['T1620.script'])
 test('T2740', normal, ghci_script, ['T2740.script'])
+test('T2950', normal, ghci_script, ['T2950.script'])
 
 test('getargs', extra_files(['../getargs.hs']), ghci_script, ['getargs.script'])
 test('T7386', normal, ghci_script, ['T7386.script'])
diff --git a/testsuite/tests/ghci.debugger/scripts/print020.stdout b/testsuite/tests/ghci.debugger/scripts/print020.stdout
index 991ed11c67..3614846f92 100644
--- a/testsuite/tests/ghci.debugger/scripts/print020.stdout
+++ b/testsuite/tests/ghci.debugger/scripts/print020.stdout
@@ -15,5 +15,5 @@ Breakpoint 13 activated at HappyTest.hs:237:18-35
 Stopped in Main.lexer, HappyTest.hs:228:11-19
 _result :: Bool = _
 c :: Char = '1'
-*** Ignoring breakpoint
+*** Ignoring breakpoint [HappyTest.hs:228:11-19]
 _result = False