docs: add faq section explaining why rclone changes fullwidth characters in file names

docs/content/faq.md

@@ -209,3 +209,21 @@ The most common cause of rclone using lots of memory is a single
 directory with thousands or millions of files in.  Rclone has to load
 this entirely into memory as rclone objects.  Each rclone object takes
 0.5k-1k of memory.
+
+### Rclone changes fullwidth Unicode punctuation marks in file names
+
+For example: On a Windows system, you have a file with name `Test：1.jpg`,
+where `：` is the Unicode fullwidth colon symbol. When using rclone
+to copy this to your Google Drive, you will notice that the file
+gets renamed to `Test:1.jpg`, where `:` is the regular (halfwidth) colon.
+
+The reason for such renames is the way rclone handles different
+[restricted filenames](/overview/#restricted-filenames) on different
+cloud storage systems. It tries to avoid ambiguous file names as
+much and allow moving files between many cloud storage systems
+transparently, by replacing invalid characters with similar looking
+Unicode characters when transferring to one storage system, and replacing
+back again when transferring to a different storage system where the
+original characters are supported. When the same Unicode characters
+are intentionally used in file names, this replacement strategy leads
+to unwanted renames. Read more [here](/overview/#restricted-filenames-caveats).

docs/content/local.md

@@ -46,6 +46,10 @@ Local file system at .: Replacing invalid UTF-8 characters in "gro\xdf"
 
 #### Restricted characters
 
+With the local backend, restrictions on the characters that are usable in
+file or directory names depend on the operating system. To check what
+rclone will replace by default on your system, run `rclone help flags local-encoding`.
+
 On non Windows platforms the following characters are replaced when
 handling file names.
 

docs/content/overview.md

@@ -138,7 +138,8 @@ Some cloud storage systems might have restrictions on the characters
 that are usable in file or directory names.
 When `rclone` detects such a name during a file upload, it will
 transparently replace the restricted characters with similar looking
-Unicode characters.
+Unicode characters. To handle the different sets of restricted characters
+for different backends, rclone uses something it calls [encoding](#encoding).
 
 This process is designed to avoid ambiguous file names as much as
 possible and allow to move files between many cloud storage systems
@@ -150,14 +151,60 @@ to ensure correct formatting and not necessarily the actual name used
 on the cloud storage.
 
 This transformation is reversed when downloading a file or parsing
-`rclone` arguments.
+`rclone` arguments. For example, when uploading a file named `my file?.txt`
-For example, when uploading a file named `my file?.txt` to Onedrive
+to Onedrive, it will be displayed as `my file?.txt` on the console, but
-will be displayed as `my file?.txt` on the console, but stored as
+stored as `my file？.txt` to Onedrive (the `?` gets replaced by the similar
-`my file？.txt` (the `?` gets replaced by the similar looking `？`
+looking `？` character, the so-called "fullwidth question mark").
-character) to Onedrive.
+The reverse transformation allows to read a file `unusual/name.txt`
-The reverse transformation allows to read a file`unusual/name.txt`
+from Google Drive, by passing the name `unusual／name.txt` on the command line
-from Google Drive, by passing the name `unusual／name.txt` (the `/` needs
+(the `/` needs to be replaced by the similar looking `／` character).
-to be replaced by the similar looking `／` character) on the command line.
+
+#### Caveats {#restricted-filenames-caveats}
+
+The filename encoding system works well in most cases, at least
+where file names are written in English or similar languages.
+You might not even notice it: It just works. In some cases it may
+lead to issues, though. E.g. when file names are written in Chinese,
+or Japanese, where it is always the Unicode fullwidth variants of the
+punctuation marks that are used.
+
+On Windows, the characters `:`, `*` and `?` are examples of restricted
+characters. If these are used in filenames on a remote that supports it,
+Rclone will transparently convert them to their fullwidth Unicode
+variants `＊`, `？` and `：` when downloading to Windows, and back again
+when uploading. This way files with names that are not allowed on Windows
+can still be stored.
+
+However, if you have files on your Windows system originally with these same
+Unicode characters in their names, they will be included in the same conversion
+process. E.g. if you create a file in your Windows filesystem with name
+`Test：1.jpg`, where `：` is the Unicode fullwidth colon symbol, and use
+rclone to upload it to Google Drive, which supports regular `:` (halfwidth
+question mark), rclone will replace the fullwidth `:` with the
+halfwidth `:` and store the file as `Test:1.jpg` in Google Drive. Since
+both Windows and Google Drive allows the name `Test：1.jpg`, it would
+probably be better if rclone just kept the name as is in this case.
+
+With the opposite situation; if you have a file named `Test:1.jpg`,
+in your Google Drive, e.g. uploaded from a Linux system where `:` is valid
+in file names. Then later use rclone to copy this file to your Windows
+computer you will notice that on your local disk it gets renamed
+to `Test：1.jpg`. The original filename is not legal on Windows, due to
+the `:`, and rclone therefore renames it to make the copy possible.
+That is all good. However, this can also lead to an issue: If you already
+had a *different* file named `Test：1.jpg` on Windows, and then use rclone
+to copy either way. Rclone will then treat the file originally named
+`Test:1.jpg` on Google Drive and the file originally named `Test：1.jpg`
+on Windows as the same file, and replace the contents from one with the other.
+
+Its virtually impossible to handle all cases like these correctly in all
+situations, but by customizing the [encoding option](#encoding), changing the
+set of characters that rclone should convert, you should be able to
+create a configuration that works well for your specific situation.
+See also the [example](/overview/#encoding-example-windows) below.
+
+(Windows was used as an example of a file system with many restricted
+characters, and Google drive a storage system with few.)
 
 #### Default restricted characters {#restricted-characters}
 
@@ -230,7 +277,7 @@ names in a different encoding than UTF-8 or UTF-16, like latin1. See the
 
 #### Encoding option {#encoding}
 
-Most backends have an encoding options, specified as a flag
+Most backends have an encoding option, specified as a flag
 `--backend-encoding` where `backend` is the name of the backend, or as
 a config parameter `encoding` (you'll need to select the Advanced
 config in `rclone config` to see it).
@@ -240,17 +287,17 @@ such a way as to preserve the maximum number of characters (see
 above).
 
 However this can be incorrect in some scenarios, for example if you
-have a Windows file system with characters such as `＊` and `？` that
+have a Windows file system with Unicode fullwidth characters
-you want to remain as those characters on the remote rather than being
+`＊`, `？` or `：`, that you want to remain as those characters on the
-translated to `*` and `?`.
+remote rather than being translated to regular (halfwidth) `*`, `?` and `:`.
 
 The `--backend-encoding` flags allow you to change that. You can
 disable the encoding completely with `--backend-encoding None` or set
 `encoding = None` in the config file.
 
 Encoding takes a comma separated list of encodings. You can see the
-list of all available characters by passing an invalid value to this
+list of all possible values by passing an invalid value to this
-flag, e.g. `--local-encoding "help"` and `rclone help flags encoding`
+flag, e.g. `--local-encoding "help"`. The command `rclone help flags encoding`
 will show you the defaults for the backends.
 
 | Encoding  | Characters |
@@ -263,7 +310,7 @@ will show you the defaults for the backends.
 | Ctl | All control characters 0x00-0x1F |
 | Del | DEL 0x7F |
 | Dollar | `$` |
-| Dot | `.` |
+| Dot | `.` or `..` as entire string |
 | DoubleQuote | `"` |
 | Hash | `#` |
 | InvalidUtf8 | An invalid UTF-8 character (e.g. latin1) |
@@ -283,6 +330,8 @@ will show you the defaults for the backends.
 | Slash | `/` |
 | SquareBracket | `[`, `]` |
 
+##### Encoding example: FTP
+
 To take a specific example, the FTP backend's default encoding is
 
     --ftp-encoding "Slash,Del,Ctl,RightSpace,Dot"
@@ -300,14 +349,42 @@ to the existing ones, giving:
 
 This can be specified using the `--ftp-encoding` flag or using an `encoding` parameter in the config file.
 
-Or let's say you have a Windows server but you want to preserve `＊`
+##### Encoding example: Windows
-and `？`, you would then have this as the encoding (the Windows
-encoding minus `Asterisk` and `Question`).
 
-    Slash,LtGt,DoubleQuote,Colon,Pipe,BackSlash,Ctl,RightSpace,RightPeriod,InvalidUtf8,Dot
+As a nother example, take a Windows system where there is a file with
+name `Test：1.jpg`, where `：` is the Unicode fullwidth colon symbol.
+When using rclone to copy this to a remote which supports `:`,
+the regular (halfwidth) colon (such as Google Drive), you will notice
+that the file gets renamed to `Test:1.jpg`.
 
-This can be specified using the `--local-encoding` flag or using an
+To avoid this you can change the set of characters rclone should convert
-`encoding` parameter in the config file.
+for the local filesystem, using command-line argument `--local-encoding`.
+Rclone's default behavior on Windows corresponds to
+
+```
+--local-encoding "Slash,LtGt,DoubleQuote,Colon,Question,Asterisk,Pipe,BackSlash,Ctl,RightSpace,RightPeriod,InvalidUtf8,Dot"
+```
+
+If you want to use fullwidth characters `：`, `＊` and `？` in your filenames
+without rclone changing them when uploading to a remote, then set the same as
+the default value but without `Colon,Question,Asterisk`:
+
+```
+--local-encoding "Slash,LtGt,DoubleQuote,Pipe,BackSlash,Ctl,RightSpace,RightPeriod,InvalidUtf8,Dot"
+```
+
+Alternatively, you can disable the conversion of any characters with `--local-encoding None`.
+
+Instead of using command-line argument `--local-encoding`, you may also set it
+as [environment variable](/docs/#environment-variables) `RCLONE_LOCAL_ENCODING`,
+or [configure](/docs/#configure) a remote of type `local` in your config,
+and set the `encoding` option there.
+
+The risk by doing this is that if you have a filename with the regular (halfwidth)
+`:`, `*` and `?` in your cloud storage, and you try to download
+it to your Windows filesystem, this will fail. These characters are not
+valid in filenames on Windows, and you have told rclone not to work around
+this by converting them to valid fullwidth variants.
 
 ### MIME Type ###