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

3 files changed, 36 insertions, 0 deletions

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);