dpkg-query: Try to use the description from the available file if not installed

When we have been requested to load the available file, and that
contains a description for a package that is not installed, we should
use that instead of printing the confusing string stating that the
description is not available.

Closes: #43573

5 files changed, 40 insertions, 2 deletions

diff --git a/debian/changelog b/debian/changelog
index b66d3c00f..2fa7c28f6 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -26,6 +26,8 @@ dpkg (1.20.0) UNRELEASED; urgency=medium
   * libdpkg, dpkg: Use new versiondescribe_c() for non-localizable call sites,
     such as when writing to a log, which should not be localized.
     Reported by Julien Cristau <jcristau@debian.org>.
+  * dpkg-query: Try to use the package synopsis from the available file if
+    not installed. Closes: #43573
   * Perl modules:
     - Dpkg::Source::Package: Verify original tarball signatures at build time.
     - Dpkg::BuildFlags: Add new unset() method.
diff --git a/lib/dpkg/libdpkg.map b/lib/dpkg/libdpkg.map
index dc58738ea..33297a8bf 100644
--- a/lib/dpkg/libdpkg.map
+++ b/lib/dpkg/libdpkg.map
@@ -300,6 +300,7 @@ LIBDPKG_PRIVATE {
 	pkg_name_const;
 	pkg_source_version;
 	pkgbin_synopsis;
+	pkg_synopsis;
 	pkg_abbrev_want;
 	pkg_abbrev_status;
 	pkg_abbrev_eflag;
diff --git a/lib/dpkg/pkg-show.c b/lib/dpkg/pkg-show.c
index 569cbf5c2..67e5af404 100644
--- a/lib/dpkg/pkg-show.c
+++ b/lib/dpkg/pkg-show.c
@@ -224,6 +224,39 @@ pkgbin_synopsis(const struct pkginfo *pkg, const struct pkgbin *pkgbin, int *len
 }
 
 /**
+ * Return a string representation of the package synopsis.
+ *
+ * The returned string must not be freed, and it's permanently allocated so
+ * can be used as long as the non-freeing memory pool has not been freed.
+ *
+ * It will try to use the installed version, otherwise it will fallback to
+ * use the available version.
+ *
+ * The package synopsis is the short description, but it is not NUL terminated,
+ * so the output len argument should be used to limit the string length.
+ *
+ * @param pkg      The package to consider.
+ * @param[out] len The length of the synopsis string within the description.
+ *
+ * @return The string representation.
+ */
+const char *
+pkg_synopsis(const struct pkginfo *pkg, int *len)
+{
+	const char *pdesc;
+
+	pdesc = pkg->installed.description;
+	if (!pdesc)
+		pdesc = pkg->available.description;
+	if (!pdesc)
+		pdesc = _("(no description available)");
+
+	*len = strcspn(pdesc, "\n");
+
+	return pdesc;
+}
+
+/**
  * Return a character abbreviated representation of the package want status.
  *
  * @param pkg The package to consider.
diff --git a/lib/dpkg/pkg-show.h b/lib/dpkg/pkg-show.h
index 9ef0ed459..4ac3f5d65 100644
--- a/lib/dpkg/pkg-show.h
+++ b/lib/dpkg/pkg-show.h
@@ -36,6 +36,8 @@ int pkg_sorter_by_nonambig_name_arch(const void *a, const void *b);
 
 const char *pkgbin_synopsis(const struct pkginfo *pkg,
                             const struct pkgbin *pkgbin, int *len_ret);
+const char *
+pkg_synopsis(const struct pkginfo *pkg, int *len_ret);
 int pkg_abbrev_want(const struct pkginfo *pkg);
 int pkg_abbrev_status(const struct pkginfo *pkg);
 int pkg_abbrev_eflag(const struct pkginfo *pkg);
diff --git a/src/querycmd.c b/src/querycmd.c
index 472d87f79..eba5e53ef 100644
--- a/src/querycmd.c
+++ b/src/querycmd.c
@@ -142,7 +142,7 @@ list_format_init(struct list_format *fmt, struct pkg_array *array)
     vlen = str_width(versiondescribe(&array->pkgs[i]->installed.version,
                                      vdew_nonambig));
     alen = str_width(dpkg_arch_describe(array->pkgs[i]->installed.arch));
-    pkgbin_synopsis(array->pkgs[i], &array->pkgs[i]->installed, &dlen);
+    pkg_synopsis(array->pkgs[i], &dlen);
 
     if (plen > fmt->nw)
       fmt->nw = plen;
@@ -232,7 +232,7 @@ pkg_array_list_item(struct pkg_array *array, struct pkginfo *pkg, void *pkg_data
   list_format_init(fmt, array);
   list_format_print_header(fmt);
 
-  pdesc = pkgbin_synopsis(pkg, &pkg->installed, &l);
+  pdesc = pkg_synopsis(pkg, &l);
   l = min(l, fmt->dw);
 
   list_format_print(fmt,