Add with pkgCacheGen::Essential a way to control which packages get the

essential flag:
* native is the default and will only mark packages of the main arch
* all will mark all packages which have these flag in Packages
* none will obviously do the opposite
* installed will only mark packages which are installed as essential,
  so it will behave in the same way as dpkg does it.
It is mostly needed sometimes for debugging but some users with special
needs might like to switch the mode as well under the expense to be
on their own…

apt-pkg/deb/deblistparser.cc

@@ -255,9 +255,13 @@ bool debListParser::UsePackage(pkgCache::PkgIterator Pkg,
    if (Pkg->Section == 0)
       Pkg->Section = UniqFindTagWrite("Section");
 
-   // Packages which are not from "our" arch doesn't get the essential flag
+   // Packages which are not from the "native" arch doesn't get the essential flag
+   // in the default "native" mode - it is also possible to mark "all" or "none".
+   // The "installed" mode is handled by ParseStatus(), See #544481 and friends.
    string const static myArch = _config->Find("APT::Architecture");
-   if (Pkg->Arch != 0 && myArch == Pkg.Arch())
+   string const static essential = _config->Find("pkgCacheGen::Essential", "native");
+   if ((essential == "native" && Pkg->Arch != 0 && myArch == Pkg.Arch()) ||
+       essential == "all")
       if (Section.FindFlag("Essential",Pkg->Flags,pkgCache::Flag::Essential) == false)
 	 return false;
    if (Section.FindFlag("Important",Pkg->Flags,pkgCache::Flag::Important) == false)
@@ -333,7 +337,13 @@ bool debListParser::ParseStatus(pkgCache::PkgIterator Pkg,
    const char *Stop;
    if (Section.Find("Status",Start,Stop) == false)
       return true;
-   
+
+   // UsePackage() is responsible for setting the flag in the default case
+   bool const static essential = _config->Find("pkgCacheGen::Essential", "") == "installed";
+   if (essential == true &&
+       Section.FindFlag("Essential",Pkg->Flags,pkgCache::Flag::Essential) == false)
+      return false;
+
    // Isolate the first word
    const char *I = Start;
    for(; I < Stop && *I != ' '; I++);

doc/examples/configure-index

@@ -432,6 +432,8 @@ Debug
   
 }
 
+pkgCacheGen::Essential "native"; // other modes: all, none, installed
+
 /* Whatever you do, do not use this configuration file!! Take out ONLY
    the portions you need! */
 This Is Not A Valid Config File