create PROJError and raise it in custom error handler

This handler, `log_func`, is registered in `__init__`. It simply throws 
when the level is at error, and ignores otherwise. Based on 
pyproj4/pyproj#215.

src/Proj4.jl

@@ -15,21 +15,11 @@ if !isfile(depsjl_path)
 end
 include(depsjl_path)
 
-const PROJ_LIB = Ref{String}()
-
-# Module initialization function
-function __init__()
-    # Always check your dependencies from `deps.jl`
-    check_deps()
-
-    PROJ_LIB[] = abspath(@__DIR__, "..", "deps", "usr", "share", "proj")
-    proj_context_set_search_paths(1, [PROJ_LIB[]])
-end
-
 include("projection_codes.jl") # ESRI and EPSG projection strings
 include("proj_capi.jl") # low-level C-facing functions (corresponding to src/proj_api.h)
 include("proj_common.jl")
 include("proj_c.jl")
+include("error.jl")
 
 function _version()
     m = match(r"(\d+).(\d+).(\d+),.+", _get_release())
@@ -71,5 +61,19 @@ function unsafe_loadstringlist(ptr::Ptr{Cstring})
     strings
 end
 
+const PROJ_LIB = Ref{String}()
+
+# Module initialization function
+function __init__()
+    # Always check your dependencies from `deps.jl`
+    check_deps()
+
+    # register custom error handler
+    funcptr = @cfunction(log_func, Ptr{Cvoid}, (Ptr{Cvoid}, Cint, Cstring))
+    proj_log_func(C_NULL, funcptr)
+
+    PROJ_LIB[] = abspath(@__DIR__, "..", "deps", "usr", "share", "proj")
+    proj_context_set_search_paths(1, [PROJ_LIB[]])
+end
 
 end # module

src/error.jl

@@ -0,0 +1,22 @@
+"Exception type based on PROJ error handling"
+struct PROJError <: Exception
+    msg::String
+    # reset PROJ's error stack on construction
+    function PROJError(msg)
+        proj_errno_reset(C_NULL)
+        new(msg)
+    end
+end
+
+function Base.showerror(io::IO, err::PROJError)
+    err = string("PROJError: ", err.msg)
+    println(io, err)
+end
+
+"Custom error handler, automatically set with `proj_log_func`"
+function log_func(user_data::Ptr{Cvoid}, level::Cint, msg::Cstring)
+    if level == PJ_LOG_ERROR
+        throw(PROJError(unsafe_string(msg)))
+    end
+    return C_NULL
+end

test/proj6api.jl

@@ -1,6 +1,27 @@
 using Test
 import Proj4
 
+@testset "Error handling" begin
+    @test_throws ArgumentError Proj4.proj_errno_string(0)
+    @test Proj4.proj_errno_string(1) == "Operation not permitted"
+    @test Proj4.proj_errno_string(2) == "No such file or directory"
+
+    # this throws an internal error that is handled by our log_func
+    @test_throws Proj4.PROJError Proj4.proj_create("+proj=bobbyjoe")
+    # ensure we have reset the error state
+    @test Proj4.proj_context_errno() == 0
+
+end
+
+@testset "Database" begin
+    crs = Proj4.proj_create_from_database("EPSG", "4326", Proj4.PJ_CATEGORY_CRS, false, C_NULL)
+    Proj4.proj_errno(crs)
+    Proj4.proj_get_id_code(crs, 0)
+    # The following is wrong, but unfortunately segfaults. How to fix other than adding
+    # a null check before the ccall in this and other functions?
+    # Proj4.proj_get_id_code(C_NULL, 0)
+end
+
 @testset "New tests" begin
 
 pj_latlon = Proj4.proj_create("EPSG:4326")
@@ -15,8 +36,8 @@ end
 
 @testset "Transformation between CRS" begin
     # taken from http://osgeo-org.1560.x6.nabble.com/PROJ-PROJ-6-0-0-proj-create-operation-factory-context-behavior-td5403470.html
-    src = Proj4.proj_create("IGNF:REUN47GAUSSL")   # area : 55.17,-21.42,55.92,-20.76 
-    tgt = Proj4.proj_create("IGNF:RGAF09UTM20")    # area : -63.2,14.25,-60.73,18.2 
+    src = Proj4.proj_create("IGNF:REUN47GAUSSL")   # area : 55.17,-21.42,55.92,-20.76
+    tgt = Proj4.proj_create("IGNF:RGAF09UTM20")    # area : -63.2,14.25,-60.73,18.2
     factory = Proj4.proj_create_operation_factory_context(C_NULL)
     @test factory != C_NULL
     @test Proj4.proj_context_errno() == 0
@@ -49,4 +70,4 @@ end
     Proj4.proj_destroy(src)
     Proj4.proj_destroy(tgt)
     # proj_context_destroy(c)
-end
+end

test/runtests.jl

@@ -1,15 +1,15 @@
 using Proj4
 using Test
 
-include("proj6api.jl")
-
 @testset "Proj4" begin
 
 println("""
 C library version: $(Proj4.version)  [\"$(Proj4._get_release())\"]
 geodesic support: $(Proj4.has_geodesic_support)
 """)
 
+include("proj6api.jl")
+
 # Some very basic sanity checking
 wgs84 = Projection("+proj=longlat +datum=WGS84 +no_defs")
 utm56 = Projection("+proj=utm +zone=56 +south +datum=WGS84 +units=m +no_defs")