From 9ecfa10db4a4856d445910d440aa38ee5b2b3d26 Mon Sep 17 00:00:00 2001
From: Boris Kolpackov <boris@codesynthesis.com>
Date: Wed, 25 Nov 2015 12:51:49 +0200
Subject: Implement help pager

---
 bpkg/bpkg.cxx |  19 +++--------
 bpkg/help.cxx | 106 +++++++++++++++++++++++++++++++++++++++++++++++++---------
 2 files changed, 95 insertions(+), 30 deletions(-)

diff --git a/bpkg/bpkg.cxx b/bpkg/bpkg.cxx
index 386d284..e1cdce2 100644
--- a/bpkg/bpkg.cxx
+++ b/bpkg/bpkg.cxx
@@ -85,10 +85,7 @@ try
   }
 
   if (o.help ())
-  {
-    help (help_options (), "", nullptr);
-    return 0;
-  }
+    return help (help_options (), "", nullptr);
 
   const common_options& co (o);
 
@@ -131,16 +128,10 @@ try
       // If not, then it got to be a help topic.
       //
       if (cmd_argc != 1)
-      {
-        help (ho, cmd_argv[1], nullptr);
-        return 0;
-      }
+        return help (ho, cmd_argv[1], nullptr);
     }
     else
-    {
-      help (ho, "", nullptr);
-      return 0;
-    }
+      return help (ho, "", nullptr);
   }
 
   // Handle commands.
@@ -162,7 +153,7 @@ try
     // if (cmd.pkg_verify ())
     // {
     //  if (h)
-    //    help (ho, "pkg-verify", pkg_verify_options::print_usage);
+    //    r = help (ho, "pkg-verify", pkg_verify_options::print_usage);
     //  else
     //    r = pkg_verify (parse<pkg_verify_options> (co, args), args);
     //
@@ -173,7 +164,7 @@ try
     if (cmd.NP##CMD ())                                                 \
     {                                                                   \
       if (h)                                                            \
-        help (ho, SP#CMD, print_bpkg_##NP##CMD##_usage);                \
+        r = help (ho, SP#CMD, print_bpkg_##NP##CMD##_usage);            \
       else                                                              \
         r = NP##CMD (parse<NP##CMD##_options> (co, args), args);        \
                                                                         \
diff --git a/bpkg/help.cxx b/bpkg/help.cxx
index df4a286..31a7cdd 100644
--- a/bpkg/help.cxx
+++ b/bpkg/help.cxx
@@ -4,8 +4,19 @@
 
 #include <bpkg/help>
 
+#ifndef _WIN32
+#  include <unistd.h>    // close()
+#else
+#  include <io.h>        // _close()
+#endif
+
+#include <chrono>
+#include <thread>    // this_thread::sleep_for()
 #include <iostream>
 
+#include <butl/process>
+#include <butl/fdstream>
+
 #include <bpkg/types>
 #include <bpkg/utility>
 #include <bpkg/diagnostics>
@@ -13,32 +24,95 @@
 #include <bpkg/bpkg-options>
 
 using namespace std;
+using namespace butl;
 
 namespace bpkg
 {
-  static void
-  help ()
+  struct pager
   {
-    print_bpkg_usage (cout);
-  }
+    pager (const string& name)
+    {
+      // First try less.
+      //
+      try
+      {
+        string prompt ("-Ps" + name + " (press q to quit, h for help)");
+
+        const char* args[] = {
+          "less",
+          "-R",                 // ANSI color
+          prompt.c_str (),
+          nullptr
+        };
+
+        p_ = process (args, -1); // Redirect child's stdin to a pipe.
+
+        // Wait a bit and see if the pager has exited before reading
+        // anything (e.g., because exec() couldn't find the program).
+        // If you know a cleaner way to handle this, let me know (no,
+        // a select()-based approach doesn't work; the pipe is buffered
+        // and therefore is always ready for writing).
+        //
+        this_thread::sleep_for (chrono::milliseconds (50));
+
+        bool r;
+        if (p_.try_wait (r))
+        {
+#ifndef _WIN32
+          ::close (p_.out_fd);
+#else
+          _close (p_.out_fd);
+#endif
+        }
+        else
+          os_.open (p_.out_fd);
+      }
+      catch (const process_error& e)
+      {
+        if (e.child ())
+          exit (1);
+      }
+    }
+
+    std::ostream&
+    stream ()
+    {
+      return os_.is_open () ? os_ : std::cout;
+    }
+
+    bool
+    wait ()
+    {
+      os_.close ();
+      return p_.wait ();
+    }
+
+    ~pager () {wait ();}
+
+  private:
+    process p_;
+    ofdstream os_;
+  };
 
   int
   help (const help_options&, const string& t, usage_function* usage)
   {
-    ostream& o (cout);
-
-    if (usage != nullptr)    // Command.
-      usage (o, cli::usage_para::none);
-    else if (t.empty ())             // General help.
-      help ();
-    else if (t == "common-options")  // Help topics.
+    if (usage == nullptr) // Not a command.
     {
-      print_bpkg_common_options_long_usage (cout);
+      if (t.empty ())             // General help.
+        usage = &print_bpkg_usage;
+      else if (t == "common-options")  // Help topics.
+        usage = &print_bpkg_common_options_long_usage;
+      else
+        fail << "unknown bpkg command/help topic '" << t << "'" <<
+          info << "run 'bpkg help' for more information";
     }
-    else
-      fail << "unknown bpkg command/help topic '" << t << "'" <<
-        info << "run 'bpkg help' for more information";
 
-    return 0;
+    pager p ("bpkg " + (t.empty () ? "help" : t));
+    usage (p.stream (), cli::usage_para::none);
+
+    // If the pager failed, assume it has issued some diagnostics.
+    //
+    return p.wait () ? 0 : 1;
   }
 }
-- 
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