Make the pager a logger wrapper

Rewrite the pager mechanism to make it a (parametrized) instance of `Logger`
rather than being a standalone thing.
The main interest of this is that we have more control over when to
actually start it and make the inner pager more quiet (only when
`writeToStdout` is called)

src/libcmd/repl.cc

@@ -29,6 +29,7 @@ extern "C" {
 #include "attr-path.hh"
 #include "store-api.hh"
 #include "log-store.hh"
+#include "loggers.hh"
 #include "common-eval-args.hh"
 #include "get-drvs.hh"
 #include "derivations.hh"

src/libmain/loggers.cc

@@ -1,5 +1,6 @@
 #include "loggers.hh"
 #include "progress-bar.hh"
+#include "paged-logger.hh"
 #include "util.hh"
 
 namespace nix {
@@ -53,4 +54,14 @@ void createDefaultLogger() {
     logger = makeDefaultLogger();
 }
 
+RunPager::RunPager()
+    : previousLogger(logger)
+{
+    logger = new PagedLogger(previousLogger);
+}
+
+RunPager::~RunPager() {
+    logger = previousLogger;
+}
+
 }

src/libmain/loggers.hh

@@ -1,6 +1,7 @@
 #pragma once
 ///@file
 
+#include "logging.hh"
 #include "types.hh"
 
 namespace nix {
@@ -18,4 +19,13 @@ void setLogFormat(const LogFormat & logFormat);
 
 void createDefaultLogger();
 
+class RunPager{
+private:
+    Logger* previousLogger;
+
+public:
+    RunPager();
+    ~RunPager();
+};
+
 }

src/libmain/paged-logger.cc

@@ -0,0 +1,90 @@
+#include "paged-logger.hh"
+
+#include <iostream>
+
+namespace nix {
+
+PagedLogger::PagedLogger(Logger * previousLogger)
+    : innerLogger(previousLogger)
+{
+}
+
+PagedLogger::~PagedLogger()
+{
+    try {
+        if (pid != -1) {
+            std::cout.flush();
+            dup2(stdout, STDOUT_FILENO);
+            pid.wait();
+        }
+    } catch (...) {
+        ignoreException();
+    }
+}
+
+void PagedLogger::startPager() {
+    if (pagerStarted) {
+        return;
+    }
+    innerLogger = makeSimpleLogger(false);
+    pagerStarted = true;
+    if (!isatty(STDOUT_FILENO)) return;
+    char * pager = getenv("NIX_PAGER");
+    if (!pager) pager = getenv("PAGER");
+    if (pager && ((std::string) pager == "" || (std::string) pager == "cat")) return;
+
+    Pipe toPager;
+    toPager.create();
+
+    pid = startProcess([&]() {
+        if (dup2(toPager.readSide.get(), STDIN_FILENO) == -1)
+            throw SysError("dupping stdin");
+        if (!getenv("LESS"))
+            setenv("LESS", "FRSXMK", 1);
+        restoreProcessContext();
+        if (pager)
+            execl("/bin/sh", "sh", "-c", pager, nullptr);
+        execlp("pager", "pager", nullptr);
+        execlp("less", "less", nullptr);
+        execlp("more", "more", nullptr);
+        throw SysError("executing '%1%'", pager);
+    });
+
+    pid.setKillSignal(SIGINT);
+
+    pid.setKillSignal(SIGINT);
+    stdout = fcntl(STDOUT_FILENO, F_DUPFD_CLOEXEC, 0);
+    if (dup2(toPager.writeSide.get(), STDOUT_FILENO) == -1)
+        throw SysError("dupping stdout");
+}
+
+void PagedLogger::writeToStdout(std::string_view s)
+{
+    startPager();
+    innerLogger->writeToStdout(s);
+}
+
+
+void PagedLogger::stop() { return innerLogger->stop(); }
+bool PagedLogger::isVerbose() { return false; }
+void PagedLogger::log(Verbosity lvl, const std::string_view s)
+{ innerLogger->log(lvl, s); }
+void PagedLogger::logEI(const ErrorInfo & ei) { innerLogger->logEI(ei); }
+void PagedLogger::warn(const std::string & msg)
+{ innerLogger->warn(msg);  }
+void PagedLogger::startActivity(
+    ActivityId act,
+    Verbosity lvl,
+    ActivityType type,
+    const std::string & s,
+    const Fields & fields,
+    ActivityId parent)
+{ innerLogger->startActivity(act, lvl, type, s, fields, parent); }
+void PagedLogger::stopActivity(ActivityId act)
+{ innerLogger->stopActivity(act); }
+void PagedLogger::result(ActivityId act, ResultType type, const Fields & fields)
+{ innerLogger->result(act, type, fields); }
+std::optional<char> PagedLogger::ask(std::string_view s)
+{ return innerLogger->ask(s); }
+
+}

src/libmain/paged-logger.hh

@@ -0,0 +1,34 @@
+#include "logging.hh"
+#include "util.hh"
+
+namespace nix {
+
+class PagedLogger : public Logger {
+    Logger * innerLogger;
+    bool pagerStarted = false;
+    Pid pid;
+    int stdout;
+
+    void startPager();
+
+public:
+    PagedLogger(Logger * previousLogger);
+
+    ~PagedLogger();
+
+    void writeToStdout(std::string_view s) override;
+
+    // All these methods are just forwarded to the inner logger
+
+    void stop() override;
+    bool isVerbose() override;
+    void log(Verbosity lvl, std::string_view s) override;
+    void logEI(const ErrorInfo &ei) override;
+    void warn(const std::string &msg) override;
+    void startActivity(ActivityId act, Verbosity lvl, ActivityType type, const std::string &s, const Fields &fields, ActivityId parent) override;
+    void stopActivity(ActivityId act) override;
+    void result(ActivityId act, ResultType type, const Fields &fields) override;
+    std::optional<char> ask(std::string_view s) override;
+};
+
+}

src/libmain/shared.cc

@@ -10,7 +10,6 @@
 #include <cctype>
 #include <exception>
 #include <iostream>
-
 #include <cstdlib>
 #include <sys/time.h>
 #include <sys/stat.h>
@@ -351,55 +350,6 @@ int handleExceptions(const std::string & programName, std::function<void()> fun)
     return 0;
 }
 
-
-RunPager::RunPager()
-{
-    if (!isatty(STDOUT_FILENO)) return;
-    char * pager = getenv("NIX_PAGER");
-    if (!pager) pager = getenv("PAGER");
-    if (pager && ((std::string) pager == "" || (std::string) pager == "cat")) return;
-
-    stopProgressBar();
-
-    Pipe toPager;
-    toPager.create();
-
-    pid = startProcess([&]() {
-        if (dup2(toPager.readSide.get(), STDIN_FILENO) == -1)
-            throw SysError("dupping stdin");
-        if (!getenv("LESS"))
-            setenv("LESS", "FRSXMK", 1);
-        restoreProcessContext();
-        if (pager)
-            execl("/bin/sh", "sh", "-c", pager, nullptr);
-        execlp("pager", "pager", nullptr);
-        execlp("less", "less", nullptr);
-        execlp("more", "more", nullptr);
-        throw SysError("executing '%1%'", pager);
-    });
-
-    pid.setKillSignal(SIGINT);
-    stdout = fcntl(STDOUT_FILENO, F_DUPFD_CLOEXEC, 0);
-    if (dup2(toPager.writeSide.get(), STDOUT_FILENO) == -1)
-        throw SysError("dupping stdout");
-    setLogFormat(LogFormat::raw);
-}
-
-
-RunPager::~RunPager()
-{
-    try {
-        if (pid != -1) {
-            std::cout.flush();
-            dup2(stdout, STDOUT_FILENO);
-            pid.wait();
-        }
-    } catch (...) {
-        ignoreException();
-    }
-}
-
-
 PrintFreed::~PrintFreed()
 {
     if (show)

src/libmain/shared.hh

@@ -84,21 +84,6 @@ struct LegacyArgs : public MixCommonArgs
  */
 void showManPage(const std::string & name);
 
-/**
- * The constructor of this class starts a pager if stdout is a
- * terminal and $PAGER is set. Stdout is redirected to the pager.
- */
-class RunPager
-{
-public:
-    RunPager();
-    ~RunPager();
-
-private:
-    Pid pid;
-    int stdout;
-};
-
 extern volatile ::sig_atomic_t blockInt;
 
 

src/nix-env/nix-env.cc

@@ -15,6 +15,7 @@
 #include "value-to-json.hh"
 #include "xml-writer.hh"
 #include "legacy.hh"
+#include "loggers.hh"
 
 #include <cerrno>
 #include <ctime>

src/nix-store/nix-store.cc

@@ -15,6 +15,7 @@
 #include "graphml.hh"
 #include "legacy.hh"
 #include "path-with-outputs.hh"
+#include "loggers.hh"
 
 #include <iostream>
 #include <algorithm>

src/nix/flake.cc

@@ -6,6 +6,7 @@
 #include "eval-inline.hh"
 #include "flake/flake.hh"
 #include "get-drvs.hh"
+#include "loggers.hh"
 #include "store-api.hh"
 #include "derivations.hh"
 #include "outputs-spec.hh"
@@ -1130,7 +1131,9 @@ struct CmdFlakeShow : FlakeCommand, MixJSON
             Activity act(*logger, lvlInfo, actUnknown,
                 fmt("evaluating '%s'", concatStringsSep(".", attrPathS)));
 
-            std::optional<RunPager> pager = json ? std::nullopt : std::optional<RunPager>(RunPager());
+            std::optional<RunPager> pager = json
+                ? std::nullopt
+                : std::make_optional<RunPager>();
             try {
                 auto recurse = [&]()
                 {

src/nix/log.cc

@@ -4,6 +4,7 @@
 #include "store-api.hh"
 #include "log-store.hh"
 #include "progress-bar.hh"
+#include "loggers.hh"
 
 using namespace nix;
 

src/nix/repl.cc

@@ -2,6 +2,7 @@
 #include "globals.hh"
 #include "command.hh"
 #include "installable-value.hh"
+#include "loggers.hh"
 #include "repl.hh"
 
 namespace nix {

src/nix/search.cc

@@ -9,6 +9,7 @@
 #include "eval-cache.hh"
 #include "attr-path.hh"
 #include "hilite.hh"
+#include "loggers.hh"
 
 #include <regex>
 #include <fstream>

src/nix/why-depends.cc

@@ -3,6 +3,7 @@
 #include "progress-bar.hh"
 #include "fs-accessor.hh"
 #include "shared.hh"
+#include "loggers.cc"
 
 #include <queue>