Only accept iconv implementations with the GNU behavior

This ensures the package behave more consistently across platforms,
and makes it possible to implement character fallback for invalid
sequences. This is what GLib does by requiring e.g. libiconv on BSDs.

Author: nalimilan

deps/build.jl

@@ -2,9 +2,37 @@ using BinDeps
 
 @BinDeps.setup
 
+# Check for an iconv implementation with the GNU (non-POSIX) behavior:
+# EILSEQ is returned when a sequence cannot be converted to target encoding,
+# instead of succeeding and only returning the number of invalid conversions
+# This non-standard behavior is required to allow replacing invalid sequences
+# with a user-defined character.
+# Implementations with this behavior include glibc, GNU libiconv (on which Mac
+# OS X's is based) and win_iconv.
+function validate_iconv(n, h)
+    # Needed to check libc
+    f = Libdl.dlsym_e(h, "iconv_open")
+    f == C_NULL && return false
+
+    cd = ccall(f, Ptr{Void}, (Cstring, Cstring), "ASCII", "UTF-8")
+    cd == Ptr{Void}(-1) && return false
+
+    s = "café"
+    a = similar(s.data)
+    inbufptr = Ref{Ptr{UInt8}}(pointer(s.data))
+    inbytesleft = Ref{Csize_t}(length(s.data))
+    outbufptr = Ref{Ptr{UInt8}}(pointer(a))
+    outbytesleft = Ref{Csize_t}(length(a))
+    ret = ccall(Libdl.dlsym_e(h, "iconv"), Csize_t,
+                (Ptr{Void}, Ptr{Ptr{UInt8}}, Ref{Csize_t}, Ptr{Ptr{UInt8}}, Ref{Csize_t}),
+                cd, inbufptr, inbytesleft, outbufptr, outbytesleft)
+    ccall(Libdl.dlsym_e(h, "iconv_close"), Void, (Ptr{Void},), cd) == -1 && return false
+
+    return ret == -1 % Csize_t && Libc.errno() == Libc.EILSEQ
+end
+
 libiconv = library_dependency("libiconv", aliases = ["libc", "iconv"],
-                              # Check whether libc provides iconv_open (as on Linux)
-                              validate = (n, h) -> Libdl.dlsym_e(h, "iconv_open") != C_NULL)
+                              validate = validate_iconv)
 
 @windows_only begin
     using WinRPM