Validate that units match between pipeline steps

As suggested in [0], steps in a pipeline are now checked for compliance. If the right side units in step n differ from the left side units in step n+1 the pipeline can't be constructed and an error is raised. [0] https://lists.osgeo.org/pipermail/grass-dev/2018-March/088123.html
OSGeo · Mar 26, 2018 · d1ebfcc · d1ebfcc
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diff --git a/docs/source/operations/pipeline.rst b/docs/source/operations/pipeline.rst
@@ -95,4 +95,16 @@ In the following the GRS80 ellipsoid will be applied to all steps.
     +step +proj=cart +inv
     +step +proj=merc
 
+**5. Units of operations must match between steps.**
 
+The output units of step *n* must match the expected input unit of step *n+1*. E.g., you can't
+pass an operation that outputs projected coordinates to an operation that expects angular units
+(degrees). An example of such a unit mismatch is displayed below.
+
+::
+
+    +proj=pipeline
+    +step +proj=merc  # Mercator outputs projected coordinates
+    +step +proj=robin # The Robinson projection expects angular input
+
+.. versionadded:: 5.1.0
diff --git a/src/PJ_pipeline.c b/src/PJ_pipeline.c
@@ -470,6 +470,19 @@ PJ *OPERATION(pipeline,0) {
         }
     }
 
+    /* Check that units are compatible between steps */
+    for (i = 1; i < nsteps; i++) {
+        enum pj_io_units right = pj_right (P->opaque->pipeline[i]);
+        enum pj_io_units left  = pj_left  (P->opaque->pipeline[i+1]);
+
+        if ( right == PJ_IO_UNITS_WHATEVER || left == PJ_IO_UNITS_WHATEVER )
+            continue;
+        if (right != left ) {
+            proj_log_error (P, "Pipeline: Mismatched units between step %d and %d", i, i+1);
+            return destructor (P, PJD_ERR_MALFORMED_PIPELINE);
+        }
+    }
+
     proj_log_trace (P, "Pipeline: %d steps built. Determining i/o characteristics", nsteps);
 
     /* Determine forward input (= reverse output) data type */

diff --git a/test/gie/4D-API_cs2cs-style.gie b/test/gie/4D-API_cs2cs-style.gie
@@ -243,4 +243,26 @@ accept      440720 3751320 0
 expect      440719.958709357 3751294.2109841 -4.44340920541435
 -------------------------------------------------------------------------------
 
+-------------------------------------------------------------------------------
+Test that pipelines with unit mismatch between steps can't be constructed.
+-------------------------------------------------------------------------------
+operation   +proj=pipeline
+            +step +proj=merc
+            +step +proj=merc
+expect      failure pjd_err_malformed_pipeline
+
+operation   +proj=pipeline
+            +step +proj=latlong
+            +step +proj=merc
+            +step +proj=helmert +x=200 +y=100
+expect      failure pjd_err_malformed_pipeline
+
+operation   +proj=pipeline
+            +step +proj=merc
+            +step +proj=unitconvert +xy_in=m +xy_out=km
+accept      12          56
+expect      1335.8339   7522.963
+-------------------------------------------------------------------------------
+
+
 </gie>