Introduce automatic pager for read commands

Automatically show the output of `show`, `policy`, `list`,
`search`, `showsrc` in a pager.

The pager setup is inspired by git's pager setup. Notably,
the pager is found using APT_PAGER and PAGER variables.

We wait for the pager to be setup somewhat correctly by
using a notify pipe to figure out whether execvp() was
succesful - then the pipe will read EOF as the other end
got closed by CLOEXEC during exec - or not, then the pipe
will contain an errno.

We set up the correct handlers for signals and exit to close
the fds and wait for the pager. Notably inside the signal
handler we cannot flush our streams, only close them, so
there is some duplication.

We call the InitOutputPager() function from inside the
various Do...() functions rather than setting it up
generally in InitOutput(). Doing so allows us to first
render the progress without a pager, and then setup
the pager for the content only which improves user
experience.

When we setup a pager we also take care to disable
standard input, as we should not be prompting users
while a pager is running (the pager will be reading
from the tty directly). We do this by dup2-ing() a
/dev/null over it; if we just close()d the fd, another
open() might reuse the fd number and problems could
occur.

1 files changed, 3 insertions, 0 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index b304cc674..00f4a9bcf 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -238,6 +238,9 @@ set(CONF_DIR "${CMAKE_INSTALL_FULL_SYSCONFDIR}/apt" CACHE PATH "Your /etc/apt")
 set(LIBEXEC_DIR "${CMAKE_INSTALL_FULL_LIBEXECDIR}/apt" CACHE PATH "Your /usr/libexec/apt")
 set(BIN_DIR "${CMAKE_INSTALL_FULL_BINDIR}")
 
+# Setup the default pager and the environment
+set(DEFAULT_PAGER "pager" CACHE STRING "The default pager to use for commands that support it.")
+set(PAGER_ENV "LESS=FRX\\nMORE=FRX\\nLV=C" CACHE STRING "Environment to pass to the pager. One variable per line, with escaped newlines")
 
 # Configure our configuration headers (config.h and apti18n.h)
 configure_file(CMake/config.h.in ${PROJECT_BINARY_DIR}/include/config.h)