don't call pager in non-terminals for changelog

Most pagers are nice and default to running non-interactively if they
aren't connected to a terminal and we relied on that. On ci.debian.net
the configured pager is printing a header out of nowhere though, so if
we are printing to a non-terminal we call "cat" instead.

In the rework we also "remove" the dependency on sensible-utils in sofar
as we call some alternatives if calling the utils fail.

This seems to be the last problem preventing a "PASS" status on
ci.debian.net, so we close the associated bugreport.

Closes: 755040

2 files changed, 43 insertions, 19 deletions

diff --git a/apt-private/private-utils.cc b/apt-private/private-utils.cc
index 9547a1b75..34af83c08 100644
--- a/apt-private/private-utils.cc
+++ b/apt-private/private-utils.cc
@@ -8,45 +8,69 @@
 #include <cstdlib>
 #include <unistd.h>
 
-// DisplayFileInPager - Display File with pager        			/*{{{*/
-void DisplayFileInPager(std::string filename)
+// DisplayFileInPager - Display File with pager				/*{{{*/
+void DisplayFileInPager(std::string const &filename)
 {
-   std::string pager = _config->Find("Dir::Bin::Pager", 
-                                        "/usr/bin/sensible-pager");
-
    pid_t Process = ExecFork();
    if (Process == 0)
    {
       const char *Args[3];
-      Args[0] = pager.c_str();
       Args[1] = filename.c_str();
-      Args[2] = 0;
+      Args[2] = NULL;
+      if (isatty(STDOUT_FILENO) == 1)
+      {
+	 // likely installed, provided by sensible-utils
+	 std::string const pager = _config->Find("Dir::Bin::Pager",
+	       "sensible-pager");
+	 Args[0] = pager.c_str();
+	 execvp(Args[0],(char **)Args);
+	 // lets try some obvious alternatives
+	 Args[0] = getenv("PAGER");
+	 if (Args[0] != NULL)
+	    execvp(Args[0],(char **)Args);
+
+	 Args[0] = "pager";
+	 execvp(Args[0],(char **)Args);
+      }
+      // we could read the file ourselves, but… meh
+      Args[0] = "cat";
       execvp(Args[0],(char **)Args);
       exit(100);
    }
-         
+
    // Wait for the subprocess
-   ExecWait(Process, "sensible-pager", false);
+   ExecWait(Process, "pager", false);
 }
 									/*}}}*/
-// EditFileInSensibleEditor - Edit File with editor    			/*{{{*/
-void EditFileInSensibleEditor(std::string filename)
+// EditFileInSensibleEditor - Edit File with editor			/*{{{*/
+void EditFileInSensibleEditor(std::string const &filename)
 {
-   std::string editor = _config->Find("Dir::Bin::Editor", 
-                                        "/usr/bin/sensible-editor");
-
    pid_t Process = ExecFork();
    if (Process == 0)
    {
+      // likely installed, provided by sensible-utils
+      std::string const editor = _config->Find("Dir::Bin::Editor",
+	    "sensible-editor");
       const char *Args[3];
       Args[0] = editor.c_str();
       Args[1] = filename.c_str();
-      Args[2] = 0;
+      Args[2] = NULL;
+      execvp(Args[0],(char **)Args);
+      // the usual suspects we can try as an alternative
+      Args[0] = getenv("VISUAL");
+      if (Args[0] != NULL)
+	 execvp(Args[0],(char **)Args);
+
+      Args[0] = getenv("EDITOR");
+      if (Args[0] != NULL)
+	 execvp(Args[0],(char **)Args);
+
+      Args[0] = "editor";
       execvp(Args[0],(char **)Args);
       exit(100);
    }
-         
+
    // Wait for the subprocess
-   ExecWait(Process, "sensible-editor", false);
+   ExecWait(Process, "editor", false);
 }
 									/*}}}*/
diff --git a/apt-private/private-utils.h b/apt-private/private-utils.h
index 432699787..8ba480bd4 100644
--- a/apt-private/private-utils.h
+++ b/apt-private/private-utils.h
@@ -5,7 +5,7 @@
 
 #include <string>
 
-APT_PUBLIC void DisplayFileInPager(std::string filename);
-APT_PUBLIC void EditFileInSensibleEditor(std::string filename);
+APT_PUBLIC void DisplayFileInPager(std::string const &filename);
+APT_PUBLIC void EditFileInSensibleEditor(std::string const &filename);
 
 #endif