don't drop privileges if _apt has not enough rights

Privilege dropping breaks download/source/changelog commands as they
require the _apt user to have write permissions in the current directory,
which is e.g. the case in /tmp, but not in /root, so we disable the
privilege dropping if we deal with such a directory based on idea and
code by Michael Vogt.

The alternative would be to download always to a temp directory and move
it then done, but this breaks partial file support. To resolve this, we
could move to one of our partial/ directories, but this would require a
lock which would block root from using two of these commands in
parallel. As both seems unacceptable we instead let the user choose what
to do: Either a directory is setupped for _apt, downloading as root is
accepted or – which is potentially even better – an unprivileged user is
used for the commands.

1 files changed, 4 insertions, 1 deletions

diff --git a/cmdline/apt-helper.cc b/cmdline/apt-helper.cc
index c240008aa..27abb2013 100644
--- a/cmdline/apt-helper.cc
+++ b/cmdline/apt-helper.cc
@@ -59,7 +59,10 @@ static bool DoDownloadFile(CommandLine &CmdL)
    // we use download_uri as descr and targetfile as short-descr
    new pkgAcqFile(&Fetcher, download_uri, hash, 0, download_uri, targetfile, 
                   "dest-dir-ignored", targetfile);
-   Fetcher.Run();
+
+   // Disable drop-privs if "_apt" can not write to the target dir
+   CheckDropPrivsMustBeDisabled(Fetcher);
+
    bool Failed = false;
    if (AcquireRun(Fetcher, 0, &Failed, NULL) == false || Failed == true ||
 	 FileExists(targetfile) == false)