Added test for round-tripping non-persistable (ZPN) WCS

Also added an info-level message when we use the FITS header not the table

N.b. this code would be rewritten to check if wcsId == 0, but that would break
the symmetry with the way that Jim reads the other entries such as the Psf

src/image/ExposureInfo.cc

@@ -257,11 +257,16 @@ void ExposureInfo::_readFits(
             auto archiveWcs = archive.get<Wcs>(wcsId);
             if (archiveWcs) {
                 _wcs = archiveWcs;
+            } else {
+                pex::logging::Log::getDefaultLog().info("Empty WCS extension, using FITS header");
             }
         } catch (pex::exceptions::NotFoundError & err) {
-            pex::logging::Log::getDefaultLog().warn(
-                boost::format("Could not read WCS; setting to null: %s") % err.what()
-            );
+            auto msg = str(boost::format("Could not read WCS extension; setting to null: %s") % err.what());
+            if (_wcs) {
+                msg += " ; using WCS from FITS header";
+            }
+
+            pex::logging::Log::getDefaultLog().warn(msg);
         }
         int coaddInputsId = popInt(*metadata, "COADD_INPUTS_ID");
         try {

tests/testWcsFitsTable.py

@@ -176,6 +176,20 @@ def testExposure(self):
             wcsOut = expOut.getWcs()
             self.assertEqual(wcsIn, wcsOut)
 
+    def testWcsWhenNonPersistable(self):
+        """Test that we can round-trip a WCS even when it is not persistable"""
+        import os
+
+        fileName = os.path.join(os.path.split(__file__)[0], "data", "ZPN.fits")
+        exp = lsst.afw.image.ExposureF(fileName)
+        del fileName
+
+        self.assertFalse(exp.getWcs().isPersistable(), "Test assumes that ZPN projections are not persistable")
+
+        with utilsTests.getTempFilePath(".fits") as fileName:
+            exp.writeFits(fileName)
+            exp2 = lsst.afw.image.ExposureF(fileName)
+            self.assertEqual(exp.getWcs(), exp2.getWcs())
 #####
 
 def suite():