EDSP: add APT-Release field to Package stanzas

apt-pkg/edsp.cc

@@ -113,10 +113,22 @@ void EDSP::WriteScenarioVersion(pkgDepCache &Cache, FILE* output, pkgCache::PkgI
    else if ((Ver->MultiArch & pkgCache::Version::Same) == pkgCache::Version::Same)
       fprintf(output, "Multi-Arch: same\n");
    signed short Pin = std::numeric_limits<signed short>::min();
-   for (pkgCache::VerFileIterator File = Ver.FileList(); File.end() == false; ++File) {
-      signed short const p = Cache.GetPolicy().GetPriority(File.File());
+   std::set<string> Releases;
+   for (pkgCache::VerFileIterator I = Ver.FileList(); I.end() == false; ++I) {
+      pkgCache::PkgFileIterator File = I.File();
+      signed short const p = Cache.GetPolicy().GetPriority(File);
       if (Pin < p)
 	 Pin = p;
+      if ((File->Flags & pkgCache::Flag::NotSource) != pkgCache::Flag::NotSource) {
+	 string Release = File.RelStr();
+	 if (!Release.empty())
+	    Releases.insert(Release);
+      }
+   }
+   if (!Releases.empty()) {
+       fprintf(output, "APT-Release:\n");
+       for (std::set<string>::iterator R = Releases.begin(); R != Releases.end(); ++R)
+	   fprintf(output, " %s\n", R->c_str());
    }
    fprintf(output, "APT-Pin: %d\n", Pin);
    if (Cache.GetCandidateVer(Pkg) == Ver)

doc/external-dependency-solver-protocol.txt

@@ -209,6 +209,15 @@ field. The following fields are supported in package stanzas:
   should be removed by the solver only when the Autoremove action is
   requested (see Request section).
 
+- **APT-Release:** (optional) The releases the package belongs to, according to
+  APT. The format of this field is multiline with one value per line and the
+  first line (the one containing the field name) empty. Each subsequent line
+  corresponds to one of the releases the package belongs to and looks like
+  this: `o=Debian,a=unstable,n=sid,l=Debian,c=main`. That is, each release line
+  is a comma-separated list of "key=value" pairs, each of which denotes a
+  Release file entry (Origin, Label, Codename, etc.) in the format of
+  APT_PREFERENCES(5).
+
 ### Answer
 
 An answer from the external solver to APT is either a *solution* or an