Merge branch 'pager' into 'main'

Introduce automatic pager for read commands

See merge request apt-team/apt!410

7 files changed, 189 insertions, 4 deletions

diff --git a/apt-private/private-cmndline.cc b/apt-private/private-cmndline.cc
index 5d4eeb5cb..cbf787aa9 100644
--- a/apt-private/private-cmndline.cc
+++ b/apt-private/private-cmndline.cc
@@ -509,6 +509,7 @@ static void BinarySpecificConfiguration(char const * const Binary)	/*{{{*/
       _config->CndSet("Binary::apt::APT::Keep-Downloaded-Packages", false);
       _config->CndSet("Binary::apt::APT::Get::Update::InteractiveReleaseInfoChanges", true);
       _config->CndSet("Binary::apt::APT::Cmd::Pattern-Only", true);
+      _config->CndSet("Binary::apt::Pager", true);
 
       if (isatty(STDIN_FILENO))
          _config->CndSet("Binary::apt::Dpkg::Lock::Timeout", -1);
diff --git a/apt-private/private-list.cc b/apt-private/private-list.cc
index eee657c46..6536c8df7 100644
--- a/apt-private/private-list.cc
+++ b/apt-private/private-list.cc
@@ -131,6 +131,10 @@ bool DoList(CommandLine &Cmd)
    GetLocalitySortedVersionSet(CacheFile, &bag, matcher, &progress);
    bool const ShowAllVersions = _config->FindB("APT::Cmd::All-Versions", false);
    std::map<std::string, std::string> output_map;
+
+   if (not InitOutputPager())
+      return false;
+
    for (LocalitySortedVersionSet::iterator V = bag.begin(); V != bag.end(); ++V)
    {
       std::stringstream outs;
diff --git a/apt-private/private-main.cc b/apt-private/private-main.cc
index 39ad07b36..7b4f05a50 100644
--- a/apt-private/private-main.cc
+++ b/apt-private/private-main.cc
@@ -6,6 +6,7 @@
 #include <apt-pkg/strutl.h>
 
 #include <apt-private/private-main.h>
+#include <apt-private/private-output.h>
 
 #include <iostream>
 #include <locale>
@@ -78,8 +79,8 @@ void CheckIfCalledByScript(int argc, const char *argv[])		/*{{{*/
 {
    if (unlikely(argc < 1)) return;
 
-   if(!isatty(STDOUT_FILENO) &&
-      _config->FindB("Apt::Cmd::Disable-Script-Warning", false) == false)
+   if (not IsStdoutAtty() &&
+       _config->FindB("Apt::Cmd::Disable-Script-Warning", false) == false)
    {
       // NOTE: CLI interface is redundant on the I/interface, this is
       // intentional to make it easier to read.
diff --git a/apt-private/private-output.cc b/apt-private/private-output.cc
index 79a97128c..e3f795cf3 100644
--- a/apt-private/private-output.cc
+++ b/apt-private/private-output.cc
@@ -6,6 +6,7 @@
 #include <apt-pkg/configuration.h>
 #include <apt-pkg/depcache.h>
 #include <apt-pkg/error.h>
+#include <apt-pkg/fileutl.h>
 #include <apt-pkg/pkgcache.h>
 #include <apt-pkg/pkgrecords.h>
 #include <apt-pkg/policy.h>
@@ -18,11 +19,13 @@
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
+#include <fcntl.h>
 #include <iomanip>
 #include <iostream>
 #include <langinfo.h>
 #include <regex.h>
 #include <sys/ioctl.h>
+#include <sys/wait.h>
 #include <termios.h>
 #include <unistd.h>
 
@@ -41,6 +44,14 @@ std::ofstream devnull("/dev/null");
 
 
 unsigned int ScreenWidth = 80 - 1; /* - 1 for the cursor */
+static pid_t Pager = -1;
+static constexpr std::array<int, 5> PagerSignalsToHandle = {
+   SIGINT,
+   SIGHUP,
+   SIGTERM,
+   SIGQUIT,
+   SIGPIPE,
+};
 
 // SigWinch - Window size change signal handler				/*{{{*/
 // ---------------------------------------------------------------------
@@ -56,9 +67,162 @@ static void SigWinch(int)
 #endif
 }
 									/*}}}*/
+
+bool IsStdoutAtty()
+{
+   static bool is = isatty(STDOUT_FILENO);
+   return is;
+}
+
+static void WaitPager()
+{
+   if (Pager != -1)
+   {
+      c0out.flush();
+      c1out.flush();
+      c2out.flush();
+      std::cerr.flush();
+      std::cout.flush();
+      fflush(stdout);
+      fflush(stderr);
+      close(STDOUT_FILENO);
+      close(STDERR_FILENO);
+      waitpid(Pager, nullptr, 0);
+      Pager = -1;
+   }
+}
+
+static void SignalPager(int signo)
+{
+   if (Pager != -1)
+   {
+      // We can't flush from inside the signal handler.
+      close(STDOUT_FILENO);
+      close(STDERR_FILENO);
+      waitpid(Pager, nullptr, 0);
+      Pager = -1;
+      for (auto signalToHandle : PagerSignalsToHandle)
+	 signal(signalToHandle, SIG_DFL);
+      raise(signo);
+   }
+}
+
+static std::string DeterminePager()
+{
+   if (not IsStdoutAtty() || not _config->FindB("Pager", false))
+      return "";
+   if (auto pager = getenv("APT_PAGER"); pager)
+      return pager;
+   if (auto pager = getenv("PAGER"); pager)
+      return pager;
+   return DEFAULT_PAGER;
+}
+
+bool InitOutputPager()
+{
+   // InitOutputPager() behaves like a singleton, do not run twice-
+   static bool alreadyRan = false;
+   if (alreadyRan)
+      return true;
+   alreadyRan = true;
+
+   auto pager = DeterminePager();
+   if (pager.empty() || pager == "cat")
+      return true;
+
+   auto pagerEnv = VectorizeString(PAGER_ENV, '\n');
+
+   int pipefds[2] = {-1, -1};
+   int notifyPipe[2] = {-1, -1};
+
+   DEFER([&] () {
+      close(pipefds[0]);
+      close(pipefds[1]);
+      close(notifyPipe[0]);
+      close(notifyPipe[1]);
+   });
+
+   if (pipe(pipefds) || pipe(notifyPipe))
+      return _error->Errno("pipe", "Failed to setup pipe");
+
+   Pager = ExecFork();
+   if (Pager < 0)
+      return _error->Errno("fork", "Failed to fork");
+   if (Pager == 0)
+   {
+      // Child process
+      if (dup2(pipefds[0], STDIN_FILENO) == -1)
+	 goto err;
+      if (dup2(STDOUT_FILENO, STDERR_FILENO) == -1)
+	 goto err;
+
+      for (int sig = 1; sig < NSIG; sig++)
+	 signal(sig, SIG_DFL);
+
+      for (auto &v: pagerEnv)
+	 putenv(v.data());
+
+      {
+	 // If our pager name contains a space we need to invoke it in a shell. Boooo!
+	 char *cmd[] = {(char*)"/bin/sh", (char*)"-c", pager.data(), nullptr};
+	 if (std::none_of(pager.begin(), pager.end(), isspace_ascii))
+	 {
+	    cmd[0] = pager.data();
+	    cmd[1] = nullptr;
+	 }
+	 execvp(cmd[0], cmd);
+      }
+   err:
+      int err = errno;
+      FileFd::Write(notifyPipe[1], &err, sizeof(err));
+      _exit(128);
+   }
+
+   // Parent process.
+
+   // Figure out if we were able to execvp() the child successfully. To do so,
+   // read from the notifyPipe. If execvp() was successful, it was closed due
+   // to CLOEXEC. On failure, our child code writes an errno to it.
+   _error->PushToStack();
+   close(notifyPipe[1]);   // Need to close our write end so our read doesn't block
+   notifyPipe[1] = -1;
+   if (int err = 0; FileFd::Read(notifyPipe[0], &err, sizeof(err))) {
+      errno = err;
+      _error->WarningE("PagerSetup", "Could not execute pager");
+      _error->MergeWithStack();
+      return true;
+   }
+   _error->RevertToStack();
+
+   // When running a pager, we must not be reading from stdin/tty, so
+   // let's be safe and open /dev/null in its place.
+   if (int const nullfd = open("/dev/null", O_RDONLY); nullfd != -1)
+   {
+      dup2(nullfd, STDIN_FILENO);
+      close(nullfd);
+   }
+
+   // Redirect the output(s) to the pager
+   if (dup2(pipefds[1], STDOUT_FILENO) == -1)
+      abort();
+   if (isatty(STDERR_FILENO) && dup2(pipefds[1], STDERR_FILENO) == -1)
+      abort();
+
+   // From now on, we can't show any progress messages as we are outputting
+   // to the pager.
+   _config->CndSet("quiet::NoUpdate", "1");
+
+   // Setup signal handlers and exit handlers for the pager, such that
+   // we wait for it.
+   for (auto signalToHandle : PagerSignalsToHandle)
+      signal(signalToHandle, SignalPager);
+   atexit(WaitPager);
+
+   return true;
+}
 bool InitOutput(std::basic_streambuf<char> * const out)			/*{{{*/
 {
-   if (!isatty(STDOUT_FILENO) && _config->FindI("quiet", -1) == -1)
+   if (not IsStdoutAtty() && _config->FindI("quiet", -1) == -1)
       _config->Set("quiet","1");
 
    c0out.rdbuf(out);
@@ -89,7 +253,7 @@ bool InitOutput(std::basic_streambuf<char> * const out)			/*{{{*/
       SigWinch(0);
    }
 
-   if (isatty(STDOUT_FILENO) == 0 || not _config->FindB("APT::Color", true) || getenv("NO_COLOR") != nullptr || getenv("APT_NO_COLOR") != nullptr)
+   if (not IsStdoutAtty() || not _config->FindB("APT::Color", true) || getenv("NO_COLOR") != nullptr || getenv("APT_NO_COLOR") != nullptr)
    {
       _config->Set("APT::Color", false);
       _config->Set("APT::Color::Highlight", "");
diff --git a/apt-private/private-output.h b/apt-private/private-output.h
index 8e1d47225..aaa9a8ed9 100644
--- a/apt-private/private-output.h
+++ b/apt-private/private-output.h
@@ -26,6 +26,8 @@ APT_PUBLIC extern std::ofstream devnull;
 APT_PUBLIC extern unsigned int ScreenWidth;
 
 APT_PUBLIC bool InitOutput(std::basic_streambuf<char> * const out = std::cout.rdbuf());
+APT_PUBLIC bool InitOutputPager() APT_MUSTCHECK;
+APT_PUBLIC bool IsStdoutAtty();
 
 void ListSingleVersion(pkgCacheFile &CacheFile, pkgRecords &records,
                        pkgCache::VerIterator const &V, std::ostream &out,
diff --git a/apt-private/private-search.cc b/apt-private/private-search.cc
index 19a3bf0fa..d6654eb5e 100644
--- a/apt-private/private-search.cc
+++ b/apt-private/private-search.cc
@@ -169,6 +169,9 @@ static bool FullTextSearch(CommandLine &CmdL)				/*{{{*/
    APT_FREE_PATTERNS();
    progress.Done();
 
+   if (not InitOutputPager())
+      return false;
+
    // FIXME: SORT! and make sorting flexible (alphabetic, by pkg status)
    // output the sorted map
    std::map<std::string, std::string>::const_iterator K;
diff --git a/apt-private/private-show.cc b/apt-private/private-show.cc
index ceef6707e..58c5d92c9 100644
--- a/apt-private/private-show.cc
+++ b/apt-private/private-show.cc
@@ -351,6 +351,10 @@ bool ShowPackage(CommandLine &CmdL)					/*{{{*/
 
    int const ShowVersion = _config->FindI("APT::Cache::Show::Version", 1);
    pkgRecords Recs(CacheFile);
+
+   if (not InitOutputPager())
+      return false;
+
    for (APT::VersionList::const_iterator Ver = verset.begin(); Ver != verset.end(); ++Ver)
    {
       pkgCache::VerFileIterator Vf;
@@ -441,6 +445,9 @@ bool ShowSrcPackage(CommandLine &CmdL)					/*{{{*/
    if (_error->PendingError() == true)
       return false;
 
+   if (not InitOutputPager())
+      return false;
+
    bool found = false;
    // avoid showing identical records
    std::set<std::string> seen;
@@ -496,6 +503,9 @@ bool Policy(CommandLine &CmdL)
    if (unlikely(Plcy == nullptr))
       return false;
 
+   if (not InitOutputPager())
+      return false;
+
    // Print out all of the package files
    if (CmdL.FileList[1] == 0)
    {