path: root/src/libpcp_fault/pmfault.3

'\"macro stdmacro
.\"
.\" Copyright (c) 2011 Ken McDonell.  All Rights Reserved.
.\" 
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.TH PMFAULT 3 "" "Performance Co-Pilot"
.SH NAME
\f3__pmFaultInject\f1,
\f3PM_FAULT_POINT\f1,
\f3PM_FAULT_CHECK\f1,
\f3__pmFaultSummary\f1 \- Fault Injection Infrastracture for QA
.SH "C SYNOPSIS"
.ft 3
#include <pcp/pmapi.h>
.br
#include <pcp/impl.h>
.br
#include <pcp/fault.h>
.sp
void __pmFaultInject(const char *\fIident\fP, int \fIclass\fP);
.br
void __pmFaultSummary(FILE *\fIf\fP);
.sp
PM_FAULT_POINT(\fIident\fP, \fIclass\fP);
.br
PM_FAULT_CHECK(\fIclass\fP);
.sp
cc \-DPM_FAULT_INJECTION=1 ... \-lpcp_fault
.ft 1
.SH DESCRIPTION
.PP
As part of the coverage-driven changes to QA in PCP 3.6, it became
apparent that we needed someway to exercise the ``uncommon''
code paths associated with error detection and recovery.
.PP
The facilities described below provide
a basic fault injection infrastructure (for
.I libpcp
only at this stage, alhough the mechanism is far more general and could
easily be extended).
.PP
A special build is required to create
.I libpcp_fault
and the associated
.I <pcp/fault.h>
header file.
Once this has been done, new QA applications may be built with
.B \-DPM_FAULT_INJECTION=1
and/or existing applications can be exercised in presence of 
fault injection by forcing
.I libpcp_fault
to be used in preference to
.I libpcp
as described below.
.PP
In the code to be tested,
.B __pmFaultInject
defines a fault point at which a fault of type
.I class
may be injected.
.I ident
is a string to uniquely identify the fault point across all
of the PCP source code, so something
like "libpcp/" __FILE__ ":<number>" works just fine.
The
.I ident
string also determines if a fault will be injected at run-time or not
\- refer to the
.B "RUN-TIME CONTROL"
section below.
.I class
selects a failure type, using one of the following defined
values (this list may well grow over time):
.TP
.B PM_FAULT_ALLOC
Will cause the
.B next
call to
.BR malloc (3),
.BR realloc (3)
or
.BR strdup (3)
to fail, returning NULL and setting
.I errno
to
.BR ENOMEM .
We could extend the coverage to all of the malloc-related routines,
but these three are sufficient to cover the vast majority of the uses
within
.IR libpcp .
.TP
.B PM_FAULT_PMAPI
Will cause the
.B next
call to a PMAPI routine
to fail by returning the (new) PCP error code
.BR PM_ERR_FAULT .
At the
this stage, only
.BR __pmRegisterAnon (3)
has been instrumented as a proof of concept for this part of the
facility.
.PP
To allow fault injection to co-exist within the production source
code,
.B PM_FAULT_POINT
is a macro that emits no code by default, but when
.B PM_FAULT_INJECTION
is defined this becomes a call to
.BR __pmFaultInject .
Throughout
.I libpcp
we use
.B PM_FAULT_POINT
and
.B not
.B __pmFaultInject
so that both
.I libpcp
and
.I libpcp_fault
can be built from the same source code.
.PP
Similarly, the macro
.B PM_FAULT_CHECK
emits no code unless
.B PM_FAULT_INJECTION
is defined, in which case if a fault of type
.I class
has been armed with
.B __pmFaultInject
then, the enclosing
routine will trigger the associated error behaviour.
For the moment, this only works for the
.I class
of
.B PM_FAULT_PMAPI
where the enclosing routine will return immediately with the value
.B PM_ERR_FAULT
\- this assumes the enclosing routine is of type
.B "int foo(...)"
like all of the PMAPI routines.
.PP
A summary of fault points seen and faults injected is produced
on stdio stream
.I f
by
.BR __pmFaultSummary .
.PP
Additional tracing (via
.B \-Dfault
and
.BR DBG_TRACE_FAULT )
and a new
PMAPI error code (\c
.BR PM_ERR_FAULT )
are also defined, although
these will only ever be seen or used in
.IR libpcp_fault .
If
.B DBG_TRACE_FAULT
is set the first time
.B __pmFaultInject
is called, then
.B __pmFaultSummary
will be called automatically to report on
.I stderr
when the application exits (via
.BR atexit (3)).
.PP
Fault injection cannot be nested.  Each call to
.B __pmFaultInject
clears any previous fault injection that has been armed, but not yet
executed.
.PP
The fault injection infrastructure is
.B not
thread-safe and should only be used with applications that are
known to be single-threaded.

.SH RUN-TIME CONTROL
.PP
By default, no fault injection is enabled at run-time, even when
.B __pmFaultInject
is called.
.PP
Faults are selectively enabled using a control file, identified by the environment
variable
.BR $PM_FAULT_CONTROL ;
if this is not set, no faults are enabled.
.PP
The control file (if it exists) is read the first time
.B __pmFaultInject
is called, and
contains lines of the form:
.ti +8n
.I ident
.I op
.I number
.br
that define fault injection guards.
.PP
.I ident
is a fault point string (as defined by a call to
.BR __pmFaultInject ,
or more usually the
.B PM_FAULT_POINT
macro).  So one needs access to the
.I libpcp
source code to determine the available
.I ident
strings and their semantics.
.PP
.I op
is one of the C-style operators
.BR >= ,
.BR > ,
.BR == ,
.BR < ,
.BR <= ,
.B !=
or
.BR %
and
.I number
is an unsigned integer.
.I op
.I number
is optional and the default is
.BR ">0"
.PP
The semantics of the fault injection guards are that each time
.B __pmFaultInject
is called for a particular
.IR ident ,
a trip count is incremented (the first
trip is 1); if the C-style expression
.I tripcount
.I op 
.I number
has the
value 1 (so
.B true
for most
.IR op s,
or the remainder equals 1 for the
.B %
.IR op ),
then
a fault of the
.I class
defined for the fault point associated with
.I ident
will be armed, and executed as soon as possible.
.PP
Within the control file, blank lines are ignored and lines
beginning with # are treated as comments.
.PP
For an existing application linked with
.I libpcp
fault injection may still be used by forcing
.I libpcp_fault
to be used in the place of
.IR libpcp .
The following example shows how this might be done.
.ft CW
.nf
$ export PM_FAULT_CONTROL=/tmp/control
$ cat $PM_FAULT_CONTROL
# ok for 2 trips, then inject errors
libpcp/events.c:1  >2

$ export LD_PRELOAD=/usr/lib/libpcp_fault.so
$ pmevent -Dfault -s 3 sample.event.records
host:      localhost
samples:   3
interval:  1.00 sec
sample.event.records[fungus]: 0 event records
__pmFaultInject(libpcp/events.c:1) ntrip=1 SKIP
sample.event.records[bogus]: 2 event records
  10:46:12.413 --- event record [0] flags 0x1 (point) ---
    sample.event.param_string "fetch #0"
  10:46:12.413 --- event record [1] flags 0x1 (point) ---
    sample.event.param_string "bingo!"
__pmFaultInject(libpcp/events.c:1) ntrip=2 SKIP
sample.event.records[fungus]: 1 event records
  10:46:03.416 --- event record [0] flags 0x1 (point) ---
__pmFaultInject(libpcp/events.c:1) ntrip=3 INJECT
sample.event.records[bogus]: pmUnpackEventRecords: Cannot allocate memory
__pmFaultInject(libpcp/events.c:1) ntrip=4 INJECT
sample.event.records[fungus]: pmUnpackEventRecords: Cannot allocate memory
__pmFaultInject(libpcp/events.c:1) ntrip=5 INJECT
sample.event.records[bogus]: pmUnpackEventRecords: Cannot allocate memory
=== Fault Injection Summary Report ===
libpcp/events.c:1: guard trip>2, 5 trips, 3 faults
.fi
.ft

.SH EXAMPLES
Refer to the PCP and PCP QA source code.
.PP
.I src/libpcp/src/derive.c
uses
.BR PM_FAULT_CHECK .
.PP
.I src/libpcp/src/err.c
and
.I src/libpcp/src/events.c
use
.BR PM_FAULT_POINT .
.PP
.I src/libpcp/src/fault.c
contains all of the the underlying implementation.
.PP
.I src/libpcp_fault
contains the recipe and Makefile for creating and
installing
.I libpcp_fault
and
.IR <pcp/fault.h> .
.PP
.I QA/477
and
.I QA/478
show examples of control file use.

.SH ENVIRONMENT
.TP
.B PM_FAULT_CONTROL
Full path to the fault injection control file.
.TP
.B LD_PRELOAD
Force
.I libpcp_fault
to be used in preference to
.IR libpcp .

.SH SEE ALSO
.BR PMAPI (3)
.SH DIAGNOSTICS
.PP
Some non-recoverable errors are reported on
.IR stderr .