py: Add stream reading of n unicode chars; unicode support by default.

With unicode enabled, this patch allows reading a fixed number of
characters from text-mode streams; eg file.read(5) will read 5 unicode
chars, which can made of more than 5 bytes.

For an ASCII stream (ie no chars > 127) it only needs to do 1 read.  If
there are lots of non-ASCII chars in a stream, then it needs multiple
reads of the underlying object.

Adds a new test for this case.  Enables unicode support by default on
unix and stmhal ports.

6 files changed, 103 insertions, 4 deletions

diff --git a/py/stream.c b/py/stream.c
index 2b4410728f..4c8b8a570a 100644
--- a/py/stream.c
+++ b/py/stream.c
@@ -67,6 +67,9 @@ STATIC mp_obj_t stream_read(uint n_args, const mp_obj_t *args) {
         nlr_raise(mp_obj_new_exception_msg(&mp_type_OSError, "Operation not supported"));
     }
 
+    // What to do if sz < -1?  Python docs don't specify this case.
+    // CPython does a readall, but here we silently let negatives through,
+    // and they will cause a MemoryError.
     mp_int_t sz;
     if (n_args == 1 || ((sz = mp_obj_get_int(args[1])) == -1)) {
         return stream_readall(args[0]);
@@ -74,7 +77,90 @@ STATIC mp_obj_t stream_read(uint n_args, const mp_obj_t *args) {
 
     #if MICROPY_PY_BUILTINS_STR_UNICODE
     if (!o->type->stream_p->is_bytes) {
-        mp_not_implemented("Reading from unicode text streams by character count");
+        // We need to read sz number of unicode characters.  Because we don't have any
+        // buffering, and because the stream API can only read bytes, we must read here
+        // in units of bytes and must never over read.  If we want sz chars, then reading
+        // sz bytes will never over-read, so we follow this approach, in a loop to keep
+        // reading until we have exactly enough chars.  This will be 1 read for text
+        // with ASCII-only chars, and about 2 reads for text with a couple of non-ASCII
+        // chars.  For text with lots of non-ASCII chars, it'll be pretty inefficient
+        // in time and memory.
+
+        vstr_t vstr;
+        vstr_init(&vstr, sz);
+        mp_uint_t more_bytes = sz;
+        mp_uint_t last_buf_offset = 0;
+        while (more_bytes > 0) {
+            char *p = vstr_add_len(&vstr, more_bytes);
+            if (p == NULL) {
+                nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_MemoryError, "out of memory"));
+            }
+            int error;
+            mp_int_t out_sz = o->type->stream_p->read(o, p, more_bytes, &error);
+            if (out_sz == -1) {
+                vstr_cut_tail_bytes(&vstr, more_bytes);
+                if (is_nonblocking_error(error)) {
+                    // With non-blocking streams, we read as much as we can.
+                    // If we read nothing, return None, just like read().
+                    // Otherwise, return data read so far.
+                    // TODO what if we have read only half a non-ASCII char?
+                    if (vstr.len == 0) {
+                        vstr_clear(&vstr);
+                        return mp_const_none;
+                    }
+                    break;
+                }
+                nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_OSError, "[Errno %d]", error));
+            }
+
+            if (out_sz == 0) {
+                // Finish reading.
+                // TODO what if we have read only half a non-ASCII char?
+                vstr_cut_tail_bytes(&vstr, more_bytes);
+                break;
+            }
+
+            // count chars from bytes just read
+            for (mp_uint_t off = last_buf_offset;;) {
+                byte b = vstr.buf[off];
+                int n;
+                if (!UTF8_IS_NONASCII(b)) {
+                    // 1-byte ASCII char
+                    n = 1;
+                } else if ((b & 0xe0) == 0xc0) {
+                    // 2-byte char
+                    n = 2;
+                } else if ((b & 0xf0) == 0xe0) {
+                    // 3-byte char
+                    n = 3;
+                } else if ((b & 0xf8) == 0xf0) {
+                    // 4-byte char
+                    n = 4;
+                } else {
+                    // TODO
+                    n = 5;
+                }
+                if (off + n <= vstr.len) {
+                    // got a whole char in n bytes
+                    off += n;
+                    sz -= 1;
+                    last_buf_offset = off;
+                    if (off >= vstr.len) {
+                        more_bytes = sz;
+                        break;
+                    }
+                } else {
+                    // didn't get a whole char, so work out how many extra bytes are needed for
+                    // this partial char, plus bytes for additional chars that we want
+                    more_bytes = (off + n - vstr.len) + (sz - 1);
+                    break;
+                }
+            }
+        }
+
+        mp_obj_t ret = mp_obj_new_str_of_type(&mp_type_str, (byte*)vstr.buf, vstr.len);
+        vstr_clear(&vstr);
+        return ret;
     }
     #endif
 
diff --git a/stmhal/mpconfigport.h b/stmhal/mpconfigport.h
index 00afa989cc..95f142ca48 100644
--- a/stmhal/mpconfigport.h
+++ b/stmhal/mpconfigport.h
@@ -44,7 +44,7 @@
 */
 #define MICROPY_ENABLE_LFN          (1)
 #define MICROPY_LFN_CODE_PAGE       (437) /* 1=SFN/ANSI 437=LFN/U.S.(OEM) */
-#define MICROPY_PY_BUILTINS_STR_UNICODE (0)
+#define MICROPY_PY_BUILTINS_STR_UNICODE (1)
 #define MICROPY_PY_BUILTINS_FROZENSET (1)
 #define MICROPY_PY_SYS_EXIT         (1)
 #define MICROPY_PY_SYS_STDFILES     (1)
diff --git a/tests/run-tests b/tests/run-tests
index 71a94f946f..4b48421ded 100755
--- a/tests/run-tests
+++ b/tests/run-tests
@@ -134,7 +134,7 @@ def main():
         if args.test_dirs is None:
             if pyb is None:
                 # run PC tests
-                test_dirs = ('basics', 'micropython', 'float', 'import', 'io', 'misc')
+                test_dirs = ('basics', 'micropython', 'float', 'import', 'io', 'misc', 'unicode')
             else:
                 # run pyboard tests
                 test_dirs = ('basics', 'micropython', 'float', 'pyb', 'pybnative', 'inlineasm')
diff --git a/tests/unicode/data/utf-8_2.txt b/tests/unicode/data/utf-8_2.txt
new file mode 100644
index 0000000000..ab0eaa4e0d
--- /dev/null
+++ b/tests/unicode/data/utf-8_2.txt
@@ -0,0 +1 @@
+aαbβcγdδ
diff --git a/tests/unicode/file2.py b/tests/unicode/file2.py
new file mode 100644
index 0000000000..aca2e0e0ed
--- /dev/null
+++ b/tests/unicode/file2.py
@@ -0,0 +1,12 @@
+# test reading a given number of characters
+
+def do(mode):
+    f = open('unicode/data/utf-8_2.txt', mode)
+    print(f.read(1))
+    print(f.read(1))
+    print(f.read(2))
+    print(f.read(4))
+    f.close()
+
+do('rb')
+do('rt')
diff --git a/unix/mpconfigport.h b/unix/mpconfigport.h
index 0831e3fd34..ce4365d365 100644
--- a/unix/mpconfigport.h
+++ b/unix/mpconfigport.h
@@ -43,7 +43,7 @@
 #define MICROPY_LONGINT_IMPL        (MICROPY_LONGINT_IMPL_MPZ)
 #define MICROPY_STREAMS_NON_BLOCK   (1)
 #define MICROPY_OPT_COMPUTED_GOTO   (1)
-#define MICROPY_PY_BUILTINS_STR_UNICODE (0)
+#define MICROPY_PY_BUILTINS_STR_UNICODE (1)
 #define MICROPY_PY_BUILTINS_FROZENSET (1)
 #define MICROPY_PY_SYS_EXIT         (1)
 #define MICROPY_PY_SYS_PLATFORM     "linux"