add io.readChars overload (simpler, less error prone) (#16044)

* add simpler to use readChars overload

* use new readChars overload

* Update lib/wrappers/openssl.nim

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>
Co-authored-by: flywind <xzsflywind@gmail.com>

lib/std/sha1.nim

@@ -231,7 +231,7 @@ proc secureHashFile*(filename: string): SecureHash =
   var state = newSha1State()
   var buffer = newString(BufferLength)
   while true:
-    let length = readChars(f, buffer, 0, BufferLength)
+    let length = readChars(f, buffer)
     if length == 0:
       break
     buffer.setLen(length)

lib/system/io.nim

@@ -174,14 +174,18 @@ proc readBytes*(f: File, a: var openArray[int8|uint8], start, len: Natural): int
   ## `len` (if not as many bytes are remaining), but not greater.
   result = readBuffer(f, addr(a[start]), len)
 
+proc readChars*(f: File, a: var openArray[char]): int {.tags: [ReadIOEffect], benign.} =
+  ## reads up to `a.len` bytes into the buffer `a`. Returns
+  ## the actual number of bytes that have been read which may be less than
+  ## `a.len` (if not as many bytes are remaining), but not greater.
+  result = readBuffer(f, addr(a[0]), a.len)
+
 proc readChars*(f: File, a: var openArray[char], start, len: Natural): int {.
-  tags: [ReadIOEffect], benign.} =
+  tags: [ReadIOEffect], benign, deprecated:
+    "use other `readChars` overload, possibly via: readChars(toOpenArray(buf, start, len-1))".} =
   ## reads `len` bytes into the buffer `a` starting at ``a[start]``. Returns
   ## the actual number of bytes that have been read which may be less than
   ## `len` (if not as many bytes are remaining), but not greater.
-  ##
-  ## **Warning:** The buffer `a` must be pre-allocated. This can be done
-  ## using, for example, ``newString``.
   if (start + len) > len(a):
     raiseEIO("buffer overflow: (start+len) > length of openarray buffer")
   result = readBuffer(f, addr(a[start]), len)

lib/wrappers/openssl.nim

@@ -760,7 +760,7 @@ proc md5_File*(file: string): string {.raises: [IOError,Exception].} =
     ctx: MD5_CTX
 
   discard md5_Init(ctx)
-  while(let bytes = f.readChars(buf, 0, sz); bytes > 0):
+  while (let bytes = f.readChars(buf); bytes > 0):
     discard md5_Update(ctx, buf[0].addr, cast[csize_t](bytes))
 
   discard md5_Final(buf[0].addr, ctx)