nix repl: add :log

Add a :log command that shows logs for a derivation.

Closes https://github.com/NixOS/nix/issues/3504

Co-authored-by: Taeer Bar-Yam <taeer@bar-yam.me>

doc/manual/src/release-notes/rl-next.md

@@ -8,3 +8,5 @@
 
 * New built-in function: `builtins.groupBy`, with the same functionality as
   Nixpkgs' `lib.groupBy`, but faster.
+
+* `nix repl` now has a `:log` command.

src/nix/repl.cc

@@ -430,7 +430,8 @@ bool NixRepl::processLine(string line)
              << "  :s <expr>     Build dependencies of derivation, then start nix-shell\n"
              << "  :t <expr>     Describe result of evaluation\n"
              << "  :u <expr>     Build derivation, then start nix-shell\n"
-             << "  :doc <expr>   Show documentation of a builtin function\n";
+             << "  :doc <expr>   Show documentation of a builtin function\n"
+             << "  :log <expr>   Show logs for a derivation\n";
     }
 
     else if (command == ":a" || command == ":add") {
@@ -500,7 +501,7 @@ bool NixRepl::processLine(string line)
         runNix("nix-shell", {state->store->printStorePath(drvPath)});
     }
 
-    else if (command == ":b" || command == ":i" || command == ":s") {
+    else if (command == ":b" || command == ":i" || command == ":s" || command == ":log") {
         Value v;
         evalString(arg, v);
         StorePath drvPath = getDerivationPath(v);
@@ -514,6 +515,27 @@ bool NixRepl::processLine(string line)
                 logger->cout("  %s -> %s", outputName, state->store->printStorePath(outputPath));
         } else if (command == ":i") {
             runNix("nix-env", {"-i", drvPathRaw});
+        } else if (command == ":log") {
+            settings.readOnlyMode = true;
+            Finally roModeReset([&]() {
+                settings.readOnlyMode = false;
+            });
+            auto subs = getDefaultSubstituters();
+
+            subs.push_front(state->store);
+
+            bool foundLog = false;
+            RunPager pager;
+            for (auto & sub : subs) {
+                auto log = sub->getBuildLog(drvPath);
+                if (log) {
+                    printInfo("got build log for '%s' from '%s'", drvPathRaw, sub->getUri());
+                    logger->writeToStdout(*log);
+                    foundLog = true;
+                    break;
+                }
+            }
+            if (!foundLog) throw Error("build log of '%s' is not available", drvPathRaw);
         } else {
             runNix("nix-shell", {drvPathRaw});
         }

src/nix/repl.md

@@ -35,14 +35,17 @@ R""(
   nix-repl> emacs.drvPath
   "/nix/store/lp0sjrhgg03y2n0l10n70rg0k7hhyz0l-emacs-27.1.drv"
 
-  nix-repl> drv = runCommand "hello" { buildInputs = [ hello ]; } "hello > $out"
+  nix-repl> drv = runCommand "hello" { buildInputs = [ hello ]; } "hello; hello > $out"
 
-  nix-repl> :b x
+  nix-repl> :b drv
   this derivation produced the following outputs:
     out -> /nix/store/0njwbgwmkwls0w5dv9mpc1pq5fj39q0l-hello
 
   nix-repl> builtins.readFile drv
   "Hello, world!\n"
+
+  nix-repl> :log drv
+  Hello, world!
   ```
 
 # Description

tests/repl.sh

@@ -3,6 +3,13 @@ source common.sh
 replCmds="
 simple = import ./simple.nix
 :b simple
+:log simple
+"
+
+replFailingCmds="
+failing = import ./simple-failing.nix
+:b failing
+:log failing
 "
 
 testRepl () {
@@ -12,6 +19,12 @@ testRepl () {
     local outPath=$(echo "$replOutput" |&
         grep -o -E "$NIX_STORE_DIR/\w*-simple")
     nix path-info "${nixArgs[@]}" "$outPath"
+    # simple.nix prints a PATH during build
+    echo "$replOutput" | grep -qs 'PATH=' || fail "nix repl :log doesn't output logs"
+    local replOutput="$(nix repl "${nixArgs[@]}" <<< "$replFailingCmds")"
+    echo "$replOutput"
+    echo "$replOutput" | grep -qs 'This should fail' \
+      || fail "nix repl :log doesn't output logs for a failed derivation"
 }
 
 # Simple test, try building a drv

tests/simple-failing.builder.sh

@@ -0,0 +1,2 @@
+echo "This should fail"
+exit 1

tests/simple-failing.nix

@@ -0,0 +1,8 @@
+with import ./config.nix;
+
+mkDerivation {
+  name = "simple-failing";
+  builder = ./simple-failing.builder.sh;
+  PATH = "";
+  goodPath = path;
+}