Add a new variable `inhibit-interaction'

* doc/lispref/elisp.texi (Top): Add a link.
* doc/lispref/errors.texi (Standard Errors): Mention the new error.

* doc/lispref/minibuf.texi (Minibuffers): Add a link.
(Inhibiting Interaction): New node.

* src/data.c (syms_of_data): Define the `inhibited-interaction' error.

* src/lisp.h: Export the barfing function.

* src/lread.c (Fread_char, Fread_event, Fread_char_exclusive):
Barf if inhibited.

* src/minibuf.c (barf_if_interaction_inhibited): New function.
(Fread_from_minibuffer, Fread_no_blanks_input): Barf if inhibited.
(syms_of_minibuf): Define the `inhibit-interaction' variable.

3 files changed, 39 insertions, 0 deletions

diff --git a/doc/lispref/elisp.texi b/doc/lispref/elisp.texi
index fa548b503aa..12255d122f9 100644
--- a/doc/lispref/elisp.texi
+++ b/doc/lispref/elisp.texi
@@ -739,6 +739,7 @@ Minibuffers
 * Minibuffer Windows::      Operating on the special minibuffer windows.
 * Minibuffer Contents::     How such commands access the minibuffer text.
 * Recursive Mini::          Whether recursive entry to minibuffer is allowed.
+* Inhibiting Interaction::  Running Emacs when no interaction is possible.
 * Minibuffer Misc::         Various customization hooks and variables.
 
 Completion
diff --git a/doc/lispref/errors.texi b/doc/lispref/errors.texi
index 9ec12714991..fb393b951f1 100644
--- a/doc/lispref/errors.texi
+++ b/doc/lispref/errors.texi
@@ -230,6 +230,11 @@ The message is @samp{Wrong type argument}.  @xref{Type Predicates}.
 
 @item unknown-image-type
 The message is @samp{Cannot determine image type}.  @xref{Images}.
+
+@item inhibited-interaction
+The message is @samp{User interaction while inhibited}.  This error is
+signalled when @code{inhibit-interaction} is non-@code{nil} and a user
+interaction function (like @code{read-from-minibuffer}) is called.
 @end table
 
 @ignore    The following seem to be unused now.
diff --git a/doc/lispref/minibuf.texi b/doc/lispref/minibuf.texi
index d316c1f0602..0ce17ed571a 100644
--- a/doc/lispref/minibuf.texi
+++ b/doc/lispref/minibuf.texi
@@ -32,6 +32,7 @@ argument.
 * Minibuffer Windows::        Operating on the special minibuffer windows.
 * Minibuffer Contents::       How such commands access the minibuffer text.
 * Recursive Mini::            Whether recursive entry to minibuffer is allowed.
+* Inhibiting Interaction::    Running Emacs when no interaction is possible.
 * Minibuffer Misc::           Various customization hooks and variables.
 @end menu
 
@@ -2617,6 +2618,38 @@ to @code{t} in the interactive declaration (@pxref{Using Interactive}).
 The minibuffer command @code{next-matching-history-element} (normally
 @kbd{M-s} in the minibuffer) does the latter.
 
+@node Inhibiting Interaction
+@section Inhibiting Interaction
+
+It's sometimes useful to be able to run Emacs as a headless server
+process that responds to commands given over a network connection.
+However, Emacs is primarily a platform for interactive usage, so many
+commands prompt the user for feedback in certain anomalous situations.
+This makes this use case more difficult, since the server process will
+just hang waiting for user input.
+
+@vindex inhibit-interaction
+Binding the @code{inhibit-interaction} variable to something
+non-@code{nil} makes Emacs signal a @code{inhibited-interaction} error
+instead of prompting, which can then be used by the server process to
+handle these situations.
+
+Here's a typical use case:
+
+@lisp
+(let ((inhibit-interaction t))
+  (respond-to-client
+   (condition-case err
+       (my-client-handling-function)
+     (inhibited-interaction err))))
+@end lisp
+
+If @code{my-client-handling-function} ends up calling something that
+asks the user for something (via @code{y-or-n-p} or
+@code{read-from-minibuffer} or the like), an
+@code{inhibited-interaction} error is signalled instead.  The server
+code then catches that error and reports it to the client.
+
 @node Minibuffer Misc
 @section Minibuffer Miscellany